I feel like they're very "deep" for my foot, which could be a sizing issue
It may be, but I was actually talking to another skater recently and the deeper heel pocket in the Jacksons is one of the things I like.. and that actually has nothing to do with where the boots come to over my ankle, but I always had rubbing where my achilles attaches to the back of my heel and it was constantly irritated in SP Teri's, even my customs (which had extra padding throughout the ankle areas since I'd recently had surgery to correct an ankle problem caused by my prior pair of stock boots, which I think helped a little, but it still points out that the heel cup isn't a very good shape for my feet). I never had heel slipping in my SP Teri's (I believe I was always fit too narrow though - even win my customs), but it was just pressure on the back of my heel (particularly with deep ankle bends).
I don't get that pressure at all in my Jacksons, and because I have a split width I don't get any heel slipping either. It might be a different story if I'd gone with a stock width to match my ball width, but it's kind of obvious looking at my feet that they are much wider at the ball than the heel. For someone else though, with a differently shaped heel, the Jacksons may very well be as much of a problem for them as the SP Teri's were for me. I toughed it out because compared to what was available locally (Reidell and Harlick) it was the better of the options available to me. At the time no one locally wore Jacksons at all and there were a few skaters who traveled a couple of hours away to be fitted in Risports (which I had done for my first pair because that's where my dad lived and he paid for them, but I was told my feet were "too wide" for risports by our local fitter later on) but for the most part everyone wore either SP Teri or Harlick, and it seemed like only LTS skaters wore Reidell (which has changed, now most skaters locally wear Reidell until they reach higher FS levels).
I agree that fit is a lot of trial and error - it's not just getting fit right, but being able to identify problem areas BEFORE they become bigger problems. As an adult I know it makes more sense to cut my losses and buy another pair of boots than to potentially deal with the consequences if I am having problems with my boots. Boots are much cheaper than my insurance deductible
I am also pickier about boot fit with my kids than I think my parents were with me... my daughter's feet are miniature versions of mine and she will probably need a split width when she gets into freestyle levels... for now if her heels are slipping it doesn't seem to be a problem for her, or at least she isn't complaining about it, but as she progresses, it's definitely something I will be watching for.