I don't have much experience in the philosophy or theory of striking/kicking, nor have I taken any specific classes in it.
Basically, I have a heavy bag (100lbs) that is sand, that I got for punching, and kicking, because I like to box, but I also wanted to improve my kicks.
The bottom, as you all probably know, is very hard (it's a fucking sand bag).
I had to learn to kick only with my shin, because kicking the hard bottom part of my sand bag with my instep, hurt like a bitch, and I could not run for a courple days.
Idunno if it's just because of not having conditioned my instep, or what, but that's just my observation: That it's better to kick with the shin, atleast for leg kicks, especially if it's checked by the other persons shin/knee/elbow because those all resemble the hardness of my bag.
But, I mean, kicking with your foot will be fine on every other part I think.