Ultimately, telling players that somehing is super unimportant is technically Railroading. Not that is is a bad type of railroading (as it keeps the adventure in line), but the mere presence of the room allows the player to branch of your planned plot and investigate. Let me illustrate:
I ran a game of exploration. I did roll on a custom made table of stuff they might find anywhere, that there was an old well shaft in the area of the flats they did check out. Just the shaft, not the wall mind you. So, I presented them the next morning, just after breaking up the camp, with this: "As you march on, you pass a hole in the ground. You can't see the bottom of it and the walls seem quite smooth." Now, what I thought as being a tiny detail to color the area as once inhabited, made the players stop in their tracks, investigating the hole for about two hours. They found out it is really deep and has water at the base. They almost sunk their bard into the hole by almost forgetting to tie him to a rope before throwing him in. They were worried about sandworms, dragons and my manical laughter as I tried to cope with the sheer lunacy of the situation. They did have a fun time guessing and questioning themselves all the time about this simple hole in the ground.
In an other example, I did run a House of Horrors type adventure, using living objects and object pretenders as enemies. To mark one room as clearly off limits, I did describe it as follows: "As you enter the silent mansion everything is cleanted thoroughly. [...] The door at the left end of the hallway is a clear exception to the mansions cleanlyness: a yard from the door the floor hasn't been wiped in year, cobwebs fill the corners and dust collects in front of the door - it seems not to have been opened in a long time." I ran this adventure about 3 times by now (I use it at conventions), and up to now, no group did dare to open the door. It is a door that clearly screams "The inhabitant we look for is not behind it." After the game I did twice revral, that this is the classic room of death (Trapper, Lurker, Stunjelly walls), and the players had a good laugh: it explained their eerie feeling about the closed door.
So, Railroading rule number one is as follows:
Everything has to serve the purpose of directing the players to the plot
This one is neither good nor bad railroading, but it is clear: a room that has no purpose, just like a hole in the ground that does not aim to make the players go to the quest goal, should just not be there. A room that utterly serves no function could as well be not on the map. Or you have to make clear that the room is insignificant.
Look at the two examples above: The well example did turn the player curiosity into an unwanted pitfall, and instead of cutting into their good fun of exploring the well I did allow them to stray off the plot. All good fun, no plot advancement.
Then the Room of Death: The door alone was clearly a place where nobody has been in years. The players wanted to complete their job to find the owner of the mansion, and deducted "he's not there". They had earlier met an iron golem and an executioner's hood and were starting to become wary about the objects in the house (as intended) and took that as a cue not to explore that room.
Ask yourself: did the room of death serve a function despite not being explored? No? Actually, it did: it gave the players an eerie feeling in a house that was supposed to do so, filled the map with a room that they would deduce to be there but the door alone was deterring them. So in the end it was railroading them to the other rooms by being appearing inhospitable and dangerous. In contrast the well did just spark the curiosity about it. "what is this, where does it come from, why is it here" - All the curiosity of the players is sparked by unexplainable things.
How to deal with seemingly empty rooms?
Now, how to deal with a room that is really absolutely boring? How to avoid the "well shaft" dilemma whithout resorting to the "room of death's" Sword of Damocles-feeling? I see several ways:
- Minor loot. Present them some pretty minor loot if they search the room. Something that ammounts to mayke a few beer in the tavern to "reward" the effort and they will feel that they checked the room. It won't change the balance too much and you can rearrange some wealth to the empty rooms if you like. Somethign like "You search the room up and down, and in the corner you find a few coins covered by a thin layer of dust."
- Make it clearly boring. In this case, present the room as what it is for your adventure: an empty room. "As soon as you open the door, stale air furrounds you. Inside you see the dirty floor, the pale walls and the ceiling, nothing else. A quick search reveals, that indeed, the room is empty but for you and the air you breathe.