A heel is not absolutely necessary, fine work to deep carving can be done without one, but having said that, a heel will get you some nice flared cuts and smooth tight bends, I have to admit that from time to time I use Steve's Universal template.
I find that for good straight lines my template is easier, as when using a heel, if you tip it the tool slightly when doing a straight line that line widens at that point, and it doesn't take much to do that.
The picture below was done mainly with a heel less tool, (no heel tool used), even the initial carved cuts, by the way, I don't get any commision for template sales
Phil