I've had no difficulty with mine being vertical. It has only been operational for a couple of months so I have no long term data. Numerous posts I've read say that mounting position is not critical. I will be moving mine into a rack when I add another battery this Spring.
Batteries have "burst disks" to prevent bursting in the rare scenario where pressure builds up inside. Ideally, the vent should be oriented to the top so all the electrolyte doesn't flow out and make a hazardous mess. Will might have a teardown video on the EG4 from which you could see the cell orientation.