I also saw a stern warning against this in one of my loading manuals. They too talked about the hardness of the shot. Actually I think it is BS, like it's going to make any difference whether the shot is a little harder or not, steel is steel. BUT I learned not to fool around and take liberties with the steel shot load recipes. This is one area of reloading where I slavishly follow the manual.
Put your BBs in a Daisy and go kill some grasshoppers.