I've always heard that it stops charging when it's full. If that's the case then there's no reason to not charge overnight.
I can't find proof of that online right now, but I also can't find proof of what you were told. The biggest thing in my favor is
that this page doesn't say
anything about not charging it over night. If those people were right I'd have to assume it'd be mentioned on Apple's own site.
Since they don't mention it then I have to think that those employees didn't know what they were talking about.