Getting back to the original topic of whether you have to finish high on the follow-through, I don't think so. As a general rule you should finish long and loose (although I've certainly seen good pitchers who didn't). As long as they are long and loose, I don't really care if they're high, low or somewhere in between. I rarely dictate a finish position, preferring for pitchers to find something natural for them.
I do try to keep them from a couple of finishes, though. One is the "touch your hand to your shoulder" finish. Some people teach that to try to encourage follow-through, but it's really not a follow-through. The elbow still stops at the side. The only thing it does is put stress on the elbow that can lead to pain and injury. That's not my opinion, that's professional trainers talking.
The other finish I try to get out of them is the "pull-back." It's when the pitcher pulls her arm down, releases the ball, and the hand pulls back instead of following through forward. It's not particularly dangerous like the shoulder touch. But you can't throw as hard and it's tougher to be accurate.
Outside of those two, I can't think of any problems. If by making a muscle you mean the arm finishes with the upper arm level to the ground and the forearm pointing up, I don't see a problem with that -- as long as they're loose.