In my student days I shared houses with males and females and never once did the whole ‘toilet seat’ argument come up. I guess everyone was just willing to accept the fact that your housemates have different opinions about that (although there were a number of discussions about the time it took certain people to do the washing up after cooking!).
For the record, I leave seat and lid down for the same reason leechbabe mentioned, it just looks better that way. I also think it’s a ‘neutral’ position, if seat and lid are down then everyone is going to have to move part of the toilet to do their business, so no arguments…right?