I think there's a difference between a random store clerk you see once, and a regular worker at a place you frequent, too. I mean, we shop at the same grocery store all the time, so we see the SAME cashiers over and over. By now, we know little bits about them, and they even recognize me if I show up dressed up and without my kids. Although I wouldn't consider them my best buds or something, I feel like we have enough of a relationship to keep it from being weird to ask them basic questions about a pregnancy.
Honestly, though, I'm pretty hard to offend. People's apparent intent makes a much bigger difference to me. If they seem to mean well, then unless it's an EXCEPTIONALLY stupid or rude question, I don't think anything of it.
One thing I think is really strange, that I've never had actually happen to me, is that strangers will rub a pregnant woman's belly. I can't imagine doing that to a stranger! And I hear it happens all the time. But I must put off a "don't mess with me" vibe or something, because I'm 7 mos. pregnant with my fourth baby, and I've never, ever had that happen. (Even friends won't touch my belly. They've commented on wanting to - and, honestly, with my friends, I'd be glad for them to - but no one has.)