I'm vertically challenged and I've had a few goose eggs from the valve hitting my head during a giant stride - just looking straight ahead. If the valve is only a little off, it lifts my mask strap off my head, causing full floods or leaks.
If you have 2 cam bands, place the top cam band right at the shoulder of the tank. Once upon a time, I used to place the top cam band just above the shoulder, not really using the top cam band. Not everyone has 2 cam bands, so it doesn't really matter. I did that to make sure the valve would not hit my head, but then people think the tank's falling out.
Does your BC fit correctly and not ride up? An ill-fitting BC can do it. As mentioned, a crotch strap helps with that. Even with a crotch strap though, if the BC is too low on the tank, even by a half inch, the valve will hit my head.