Deities basically never come to the Material Plane. When they do, it has huge ramifications for that plane as well as for the Outer Planes, and that tends to cause lots of problems and anger all the other deities.
Instead, they send avatars to the Material Plane—and even that is quite rare, because even that still causes a lot of trouble. Most of the stats for “gods” in the books really are for their avatars. You can even find different stats for the same deity in different books, because really both are just avatars.
Slaying an avatar, anywhere, probably disrupts a deity’s plans for that avatar, and it takes the deity a year to make another one. It definitely does not result in the deity’s actual death.
In the case that a deity is actually physically present in the Material Plane (or anywhere else, for that matter), then death does mean death. Or, at least, nothing seems to say it doesn’t—deities are outsiders, outsiders can die, ergo death. Deities & Demigods does suggest that when making your own pantheons, it is up to you to decide whether or not gods can actually die, but we know that D&D gods usually can die so that would only apply to custom settings.