Assuming your subfloor is decent quality (doesn't have a 'D' face which means voids in one or more plies), the tub should sit okay on it and not require anything else. If your floor is not level, (don't confuse flat with level - the tub must be level!), then the tub has to be leveled, and then you may need shims. An alternative, if it doesn't sit level, is to use some mortar underneath. This would provide the leveling and spread the load. This is more common for fiberglass or acrylic tubs that don't have the rigidity of CI, but it can be used for them as well (but it's only providing leveling and not the support). The ledger board and the feet should keep things in place, but the mortar doesn't hurt.