Because of my back problem, I don't kick high round and side kick. Even front kick, it's only to my chest level. I don't stretch before doing the kicking bags, I stretch afterwards when I am all warmed up.If your workout does not involve with high kick, you can do your stretch after your workout (or you don't even need stretch). But if your workout involve with high kick, without stretching, your high kick may pull your leg muscle.
The best procedure is:
static stretch -> dynamic stretch -> high kick work out