I'm not sure, good question. On one hand it would let the battery bounce around more, but on the other hand it might help absorb some of the shock. I could see securely mounting it to springs being a challenging problem to solve though. Making sure the springs are soft enough to compress when hitting bumps but still stiff enough that they will never bottom out and cause the battery to slam down would be a challenge too.Interesting point I had not considered.
I wonder if setting the battery on a makeshift bed of coil or leaf springs, would be good for the battery longevity, especially for off road vehicles?
For instance, a dozen springs from old oil filters cut open, affixed and sandwiched to thin boards, sandwiched under the battery?
If you get a good AGM battery, I don't think you should have to worry about vibration too much. Since the plates are jammed between fiberglass mats, they aren't prone to rattling apart like they are in flooded batteries.