Theoretically they should be done on the ground so you can see how far the truck drops for a specific turn of the bolts. Practically speaking, doing the adjustment when the suspension is "unloaded" is much less likely to strip the adjuster bolts. If you have access to a lift, you could unwind one turn, drop the truck, raise it back up and do another turn, etc until you're happy with the drop. The same thing could be done with a floor jack and stands, but it would be a lot more work.
