[Alan Anderson]

There are no actual Rules that I can find, but there are plenty of guidelines and lots of good advice.

You're right, you don't want it falling out during a match. Mount it securely.

Batteries are dense and heavy. Mount them low if you want to keep your robot's center of gravity low.

Make it easily accessible and easy to pull out without tugging on the wires, because you'll be changing it often.

Keep sharp corners and bolt ends away from it. You don't want something poking a hole in it.

Mount it in whatever orientation makes the most sense for you, but try not to put it upside down. Although it will function okay in any direction, if it overheats and starts venting you really shouldn't have the vents at the bottom.