We have leveled both ways. Typically I try to get the truck as level as possible as I pull into a site. Then stabilize with either the two front or rear jacks.
I found that the hard, flat part of the front seat armrest is in the same plane as the floor of the TC.
I put my Iphone on its internal level function and place it on the armrest and very slowly back up into where we want to park. I watch the Iphone and stop at zero.
I have read where others state that if the jacks are down you MUST release the tiedowns, in case a tire goes flat during the night. That very remote possibility would leave the truck hanging by the tiedowns. I have never lost sleep over not doing that.