I personally don't think it's a good idea to figure on going for a few sessions of hypnotherapy just to contact or discover ego states or alters or any type of that... unless of course you are quite stable, and maybe have just one or two issues left to work through.
A person uses dissociation for a good reason. Generally it's because they aren't able to face something. Dragging it out ahead of it's time could cause you more issues, and put you backwards in therapy.
With self hypnosis, you will not allow yourself to uncover or drag out any information you know you aren't ready for... that's the cool part about it.
Remember, as I revert to the older definitions, there is a big difference between someone who uses hypnosis as a hypnotist, and someone who uses it with psychotherapy.
Stay safe.