When trying to decide what keys to use, remember that at the heart of it, you're creating a user-interface, or UI (GUI, for graphic user interface, as with a mouse, etc). When designing your UI, the goal should be to make it as intuitive as possible. Never, in any program or game I have ever used or even heard of, has there ever been someone using the Pause|Break key, or the NUM LOCK. That's not to say you CAN'T use them, but they are not intuitively going to be guessed by your user.
So, to make a long story short, I agree with Ross C. Try to use standard keys. Good luck with your UI design!
So, to make a long story short, I agree with Ross C. Try to use standard keys. Good luck with your UI design!