I don't really have any big issues with double dipping. I know that I do it, but I at least make a point to turn whatever item of food that I happen to be eating around before I dip it again.
The episode of Seinfeld about it was hilarious!
I suppose that since I don't have any problems with it I really don't consider it to be rude. I don't think that I've ever been called on it before. I remember a while back when I was at a family get-together I got a new scornful glaces for it. And I made a point to stop for the remainder of the day...
Plus, I suppose that its a good point that you can pass many ocky germs onto others by double dipping. But I think that if a person isn't really sick enough to say anything about it or show any symptoms, then I really don't think that there is anything to worry about.