I agree with Amber that blocking off any kind of entrance is a bad idea. There WILL be that one person, or two people, that will end up standing in that entrance space and block it from anyone else coming in. I personally wouldn't risk it.
I've never liked being behind my tables. I feel like I can't see my customers or my items they're asking about. So for the past few years I've been setting up my booth in a U shape (or L shape), as big as I could get it, and setting up a little checkout / wrapping station in one of the corners. That way I can walk around with my costumers and help them find anything they want or stay out of the way in the corner, but still involved. And talk about easy re-organizing and restocking! It's worked -really- well! :D