Anyway, my point is: you have to assume that everyone is a source of filth and germs. I had to train myself not to touch my face or rub my eyes. Keep sanitary wipes handy, and wipe off periodically, no matter who has been at your counter.
Yeah, I'll admit that I cringe internally when someone puts their purse on the counter, because odds are that the purse has been in the bathroom, and between the germs spread when flushing and the fact that people grab their purse after they've used the bathroom but before they've washed their hands, I'm pretty sure that purses are just spreading bacteria everywhere.
That, and you don't know how clean other people's hands are.
So while I don't think it's a good idea to put a kid down on a counter, I also would never, ever assume that a counter was clean unless I just wiped it down with a bleach wipe.
If I think about it too much I'd be afraid to touch anything, so instead I remind myself to wash my hands often and not touch my face. (And now I'm thinking about it and I need to go wash my hands.)