I've never even thought about it to be honest. A lot of stuff i get is from online so not sure anyone would have had the chance to try it on beforehand.
Although if your in a shop i suppose there is a chance someone has already tried on that same shirt or jeans, would the vendor wash them if someone had tried on and not purchased? I know Topman and some other places take the clothes you've tried from you if you don't want to buy them.
Unless it smells dodgy I wouldn't bother, and i don't think i ever have