I wash all clothing right away, no matter where it came from, assuming it's washable and I'm going to wear it. Mostly out of habit- that's what I grew up with doing. Also, although it sounds silly, I prefer having clothing that smells like familiar detergent to clothing that smells like store.
I alwas assumed people wash things before donation and that the places washed them, but have since been told most places don't wash the clothing, and years of retail show me how icky people can be, so I always wash first. Even if it's not icky, you don't know where it's been, and despite having a bunny, I'm allergic to most pets (including my own) so I figure it would be a good idea to do a hypoallergenic wash first. Got a great wool coat at the dry cleaners right now