I never used to care, until a guy that I've gone out on a few dates with (who lives next door to where I work, and keeps accidently bumping into me) was around, now I spend 10 mins applying "natural look" make up on before I leave the house.. ) :
I ALWAYS dress to impress, but that's just me. But those occasional days when I'm feeling tired and lazy, i just throw on some sweats.
But you're right- you never know who you're gonna run into! :0