You can place it in any orientation, the only thing I strongly recommend is centering it on the robot. Although if you’re doing it like us this year, we have it on the side to counteract our offset shooter.
Most manufacturers on the allowed list state that the battery can be used in any orientation and charged in any orientation. Personally I recommend that all batteries be charged with the terminals facing up to give the vents on top a chance to vent only gas and not liquid. As always, battery terminals need to be insulated at all times.
In case you didn’t recognize this, Al is not only an electrical/electronics professional, but THE Lead Robot Inspector for FIRST. And OBTW, I fully agree with the above: Use batteries however, charge with terminals at (or essentially at) the highest point of the battery so you vent gas rather than gelatinous acid. Charging in another orientation shortens battery life and corrodes your stuff that happens to be near the battery when you charge it.