Hmm, I'm sure guys will find a way to combat my reasoning, but this is the way I see it:
Girls need the toilet seat down 100% of the time.
Guys need the toilet seat down maybe 25% of the time (depending on the individual).
To keep it simple, that's 63% of the time that the toilet seat will need to be down (assuming there is an equal amount of girls as guys in the household). Since that's 63% versus 37%, I say keep the toilet seat down.
And in households that are made up mostly of guys, it's not unusual for the toilet seat to just stay up anyways.
EDIT: or just listen to Ledgem above me. XD