Not sure what you mean by backing board?
I regularly mount up to 24 x 36's on 1/4 gator and then 30 x 45, 40 x 60 and 36 x 72 on 1/2 inch Never back it. Just put it in a frame with a deep enough rabbet to handle the gator/canvas. I will cover the back of the frame with paper, but not the gator. I never show/sell gator canvas unless it's framed.
Paul C