>Is it a rule?
I found this rather tricky when I worked in New York for a while. I was always putting the commas and full stops(periods) in the wrong places but I was never completely sure of the difference. As I was working for a marketing department I had to have all my spelling and grammar looked over (oversighted - yuk) be a real American to make sure that the customers would not think that they had employed an idiot!!! The problem is that my UK grammar is a bit ropey too, so they may well have been right.
I think that we tend to put a comma or a full stop in different places according to whether the entire sentence is within the parenthesis or inverted commas. According to Wow's US stylebook - the AP says, "Periods always go inside quotation marks." I'm pretty sure that this was followed slavishly.
