However you decide to mount it, make sure you use foam. It will reduce the vibrations that the battery experiences. You don't want the internal plates touching each other (or any of the other bad things that can happen to a battery). Strap in or otherwise contain your battery as well. Your robot will wind up in an orientation you don't expect.
Battery orientation (flat or standing up) doesn't matter as long as it's not upside down.
Otherwise, what everyone else said applies. Low, contributing to a balanced COG, short wires that don't get pinched and mounted in a way that makes it easy to change batteries quickly.
I recommend using
90º aluminum angle to make your battery mount. It's the perfect shape, strong, light and easy to interface with your other structure.
Here's the battery mount from our 2012 robot. It uses angle on 3 sides with a strap and foam to contain the battery. It tipped over on the field several times without issue.