In my dojo you don't hear any vocal kiai's. In my old style, we had to kiai every time we attacked, every time we were attacked, every time we were thrown, every time we rolled or practiced falls, every move of our kata, etc.
I do believe that a kiai should be natural and unforced. I find that you can easily tell if someone is forcing it out or if it is just part of them. Now, my breathing habits I learned from kiaing have stuck with me, but mine are now more like silent exhalations instead.