I always kick with the bottom half of my shin, closer to the foot.
I had a cheap heavy bag that felt like it was filled with sand and the bottom was super hard. Thought I was going to break my leg. So I bought an outslayer muy thai heavy bag with no filling, and I filled it myself with carpet padding and old clothes.
Then I started kicking the shit out of it and it was nice and gentle on my shins. I keep cramming more and more clothes into it as it settles. I'd estimate that it weighs about 60 lbs now, which is heavy enough for me. I can always empty it out and fill it with something heavier if I need to.
Anyhoo, my shins are tougher now. Not nearly as tough as a real fighter but if I had to defend myself in a street fight I could throw some leg leg kicks without hurting my shins.