Two of my bigs (and my SF) are getting some guard training this season because I am training them to be PF so they need some handles, a bit of outside D and a good jump shot. I am doing it now because in div IV I can afford to play them at PG for one position training, but when I get promoted it will cost me games to play guys that far out of position. It's part of a longer training plan to try to produce a couple of really good PFs and a decent small forward, and next season there will be more bigs brought in to concentrate on rebounding.
I wouldn't try to "round out" a guard by giving them skills that aren't going to benefit them. Think of it this way: Even if an extra level of rebounding is a bit useful for a SG it is still less useful than:
a) an extra level of jump shot (or driving, or whatever) on that guard
b) an extra level of rebounding on a big man