If your going to practice your kicks using the instep of your foot then dont use a heavy bag, use something with much more give that you can kick through.
Heavy bags are best for ocnditioning the shins with MT style kicks... Which i personally prefer because they are MUCH more powerful and not a lot slower, plus your less likely to injure your leg or foot. Just look up Buakaw on Youtube.
Also, i've seen tae kwon do style kicks work in tae kwon do tournies, but never in MMA...