MI Eventer has some great points.
I was in lessons for a year and was constantly told "keep your heals down!", keeping my heals as far "down" as she wanted caused problems with future horses. I then had a very kind, smart horse woman take me aside and work with me on my seat, NOT ON MY HEALS, she dropped my stirrups way down so that I barely had them (because we were in a public riding ring not a private one or else she would have taken them totally) it taught me to rely on my butt and upper thighs to keep in the the proper position and over his center of gravity.
I thank her for that immensely now because I have a high strung TB who constantly tests me, its my seat that keeps me on his back and NOT my heals, my heals are still down, yes, because it was POUNDED into my head that they had to be, but they are no longer as far down as they used to be.
If you have a solid seat there is no reason to deal with forcing your heals down.
16 year old TWH Mare named Ginger
3 year old APHA Paint Gelding named Fox.
RIP Evie (Clydesdale) 7.29.09 & Magic (OTTB) 2.23.10