It might be optimal in many cases but is not absolutely necessary. I have seen elite players hit the IO shot on the inside as well as the outside of the ball. If the OP is having difficulty keeping the ball out of the alley, I would suggest going primarily for the outside of the ball for a while. Here is Gary Sakuma (hi10spro) hitting IO forehands. While he hits some of them on the inside of the ball, many are on the outside (particularly in the 1st part of this video):
While these may fit the technical definition of an I/O, Imo you would be far better off hitting a good backhand than these in the vid, which mainly just seem to pull him out of position without gaining much of the advantages of what imo is a real I/O forehand. I guess if you don't have a backhand, this beats hitting a very poor shot.