If the PCs use plane shift to travel to a plane to which they have an attuned tuning fork, do they need an attuned tuning fork to return to the Material Plane?
Yes, you need the attuned tuning fork.
Using plane shift, you'll need a fork attuned to the Material Plane to travel to the Material Plane. The tuning fork is not optional.
The plane shift spell says:
Components: V, S, M (a forked, metal rod worth at least 250 gp, attuned to a particular plane of existence)
A rules lawyer will say, but it doesn't say it has to be attuned to the destination plane. Good luck with that, maybe your GM will buy it. I wouldn't. And my plane-shifting wizard's GM wouldn't either, they'd just give you that GM look and say hard no.
Using the sigil sequence of a teleportation circle
The plane shift spell also says:
Alternatively, if you know the sigil sequence of a teleportation circle on another plane of existence, this spell can take you to that circle. If the teleportation circle is too small to hold all the creatures you transported, they appear in the closest unoccupied spaces next to the circle.
Some might argue that this is presenting an alternative to using an attuned fork. However, to my reading, it is presenting an alternative to the process in the preceding paragraph, in which you specify a destination in general terms and at the GM's descretion you appear "in or near that destination", and not an alternative to the attuned fork. Using a teleportation circle allows you to specify the destination more precisely, but you still need the fork.
Additional information in the DMG
The Dungeon Master's Guide provides additional information on using plane shift, under "Planar Travel":
Plane Shift. The plane shift spell has two important limitations. The first is the material component: a small, forked, metal rod (like a tuning fork) attuned to the desired planar destination. The spell requires the proper resonating frequency to home in on the correct location, and the fork must be made of the right material (sometimes a complex alloy) to focus the spell’s magic properly. Crafting the fork is expensive (at least 250 gp), but even the act of researching the correct specifications can lead to adventure. After all, not many people voluntarily travel into the depths of Carceri, so very few know what kind of tuning fork is required to get there.
Second, the spell doesn’t send the caster to a specific location unless he or she has specialized information. The sigil sequence of a teleportation circle located on another plane allows the caster to travel directly to that circle, but such knowledge is even harder to come by than the specifications of the required tuning fork. Otherwise, the spell transports the caster to a location in the general vicinity of the desired spot. Wherever the adventurers arrive, they’ll most likely still need to undertake a journey to reach the object of a planar quest.
That makes it very clear that a fork is not optional. A maximally cooperative GM may choose to interpret the spell as not requiring a tuning fork if using a sigil sequence, but that is not a very supportable rules-as-written interpretation.
Getting a fork
You might ask, how to even get a tuning fork for the Material Plane? It is up to the GM, but a fork for the Material Plane should be among the easiest to get, especially on the Material Plane itself.
In our game, the GM ruled that having grown up in a magical society and a large magical city, my wizard had no problem with getting a tuning fork for the Material Plane, and recipes for several others. Your GM may rule differently.
Don't leave home without it
As an aside, once a spellcaster is operating at levels where planar travels happen, it's wise to carry a fork for the Material Plane. My wizard always carries one. Early on she started carrying it, and shortly after having learned plane shift, she was hit by a prismatic spray and plane-shifted to what I think was Avernus. After a night of cowering in a rock-colored hut with a familiar on watch, she prepared plane shift and returned to the Material Plane. Smarter by far than wise, it was a rare moment of wisdom on her part to carry the fork. I wish she'd been thoughtful enough to bring back a rock from Hell, but at least she came back.
For an uneventful return trip, use a sigil sequence, too
Because otherwise, you'll return to a general location on the Material Plane. My wizard spent days traveling back to where the party was. The ideal is to carry a familiar item for the specific place on the Material Plane you want to go, then plane-shift to a known teleportation circle, then teleport to your final destination using the familiar item.
Other ways to get back home
Stuck on another plane without a fork?
There are other ways to get back than by using plane shift and a tuning fork.
The banishment spell
Any creature native to the Material Plane can be banished back to the Material Plane using the banishment spell. This has some real advantages; it is a 4th level spell so it's available to fairly low-level casters, it is available across a wide range of classes and subclasses, and there's no costly component. It has some disadvantages, too. You need to upcast to send more than one creature at a time, they get a save, it requires the caster to spend a minute concentrating to make it stick, and it says nothing about where on the Material Plane you return. That last one in particular could be a bit awkward.
The gate spell
The spell gate will also get you back. You can target a specific location on the Material Plane (or any other plane) and you can hold it open using concentration and get the whole party through at once. Unfortunately, it is a 9th level spell limited to clerics, sorcerers, and wizards. It also requires a 5000gp diamond, although it is at least not consumed by the spell.
If you yourself don't know gate, all is not completely lost. If you know anyone who knows the spell you could use sending to contact them and get them to open a gate to you. Aside from the obvious downside of needing high-powered associates and possibly owing big-time for the favor, this use of gate will only transport one creature, so it will either need to be repeated for every party member, or the one gated will have to return to the far plane with something useful, like an attuned tuning fork.
Portals between the planes
There are also portals here and there connecting the planes; as an example, fey crossings between the Feywild and the Material Plane, although finding them can be tough.
The Outlands ("the plane between the Outer Planes"), contains the gate-towns, each built around a portal leading to one of the Outer Planes. Within the Outlands is Sigil, the City of Doors, which according to the PHB "holds countless portals to other planes and worlds". Admittedly, attempting to get back to the Material Plane via The Outlands and Sigil are probably more a the resort of the truly plane-lost traveler than any reasonable plan, unless you happen to know the right portals.
There are nearly endlessly many other ways. A cleric may be able to use Divine Intervention. There is a magic item, the amulet of the planes that will do it, although perhaps with unintended complications. Both the Astral and the Ethereal planes can be used to travel between some other planes.
All and all, might be easier to get hit by prismatic spray and get lucky.