How important is it to seal/coat a PCB project for *normal* conditions?
Do hobbyists worry about it?  Heck no.  At best I might put a clearcoat on a homemade PCB to keep the copper from oxidizing (looks bad, but that's it).  Some people at large companies don't even worry about it.  I've put unsealed professionally made boards in test equipment that's being used in a factory floor environment.  In fact I'd say boards aren't typically sealed or protected at all unless they're going into an environment that's known to be difficult.  This might include engine controllers (grease, carbon deposits), anything going on a ship (salt spray, salt fog), any overly dusty area - particularly metal particles I'd imagine.