At ride height. If not the two bellows will rub each other as you drive and can cause failure. You wont dump the truck in a full year to cause as much rubbing as one day of driving with a lopsided bag.
if set up carefully u can get it so the bag is compressed even when dumped...even when at ride height..and a little angled at full lift...this may require your brackets to be a little angled...just takes some pre fitting and measuring