For semi-private places like: University library, Dorm restroom, workplace (office w/ plenty of restrooms throughout the building):
if it looks clean, I'll sit on it. If I have to wipe stuff off the seat, I'll wipe it clean and then cover the whole seat with a double layer of TP (strips folded over.... I'm lazy

). If they provide a seat cover, I'll use it even if the seat isn't dirty.
If it's a public place (stadium, University student center, some of the University lecture halls, any place in a public park / transit / shopping area, I'm going to do the two-layer TP thing regardless (unless it's regularly cleaned, like seemingly every hour in some airports and the janitor is still in the bathroom and the seat is positively gleaming).