You know, i never got that whole debate myself actually. Who the hell just says "fuck it" and puts all their trust into a seat without checking it? There's sending caution to the wind, and then there's just being dumb. Don't be dumb. Look where you sit. Jollybarracuda's Helpful Tips For Life.
But in all seriousness, i do always put the seat down when i'm done, along with the lid. It just looks nicer and less like a gas station restroom. Plus, when i found out all that stuff that gets sent into the air when you flush with the lid up - mother of god.