I have lived alone for years and I always put the seat and lid down. Interestingly, my female friends leave the lid up and my made friends leave the lid down, as they found it.
PS: Growing up, leaving the lid or seat up would get my brother or me called back to put it down. That included when we were outside playing with friends. We DID NOT want to hear my dad shout at the top of his lungs "Come in here and put the toilet seat down."