Steven, most the tubs I have installed were in new homes and the studs were all that were there. We attached the flanges on the tub edges to the studs. I admit that many were fiberglass or plastic, but see no difference in cast iron tubs. The only problem I wondered about was bringing the wall material down to the tub edge. With the flange under it, the material would have a "lump" in it. If I were doing the total install, I would remove some material on the back and make it fit flush all the way down. Then a seal could be applied.
if you have never made a mistake, you probably haven't done much.