I recall some conventional wisdom about creating an equilateral triangle with the front two (LR) channels and the main seating area. That is, if the main seating area is 10 feet from the screen, the the two speakers would be around 10 feet apart. (I realize this doesn't make a perfect equilateral triangle - it would actually be slightly wider apart).
Wider is typically better, but taken to an extreme, the front soundstage can begin to break up. In my listening area, I sit 12 feet from the screen. My fronts are located about 11 feet apart from each other. I find the soundstage to be wide, but full, with nice separation.
In truth, it will depend as much on the amount of space you have and the aesthetics of your set up. You wouldn't want to flank a 42 inch tv with speakers spread 12 feet apart. If the equilateral triangle approach doesn't work, try to put about a foot of space in between the speakers and the edges of the TV for an aesthetically pleasing look and a width of soundstage to match your screen.
