You lose downtravel with cranking the bars. Your truck will also ride stiffer. And if you maxed them out, you would prematurely wear the balljoints. Take 'prematurely' lightly....it'll take years before they are gone because of cranking.
I prefer to do it on the ground. If you jack the truck up, then crank, then lower truck to see how far it grew, it's an inaccurate reading of lift. You must drive down the road a few times to let the bars 'settle'. That's why i like to do it on the ground.
I think we've got a how-to on here...we must!