Whether or not you're touching the body of the case or not depends on the case. I would imagine if you're reloading one of the shorties or an AI, you can't use that method without working part of the body of the case but a lot of the others, you can. I've done it. Even if you can't, you're holding the case in better alignment.
When I'm setting up a F/L die, I set it up so it is indeed full length resizing. Then I get a peice of thin flat stock or a thin fender washer and make a hole in it that is big enough that the body of the die will fit thru it. Then I slide it onto the body of the die so it fits between the locking ring and the top of the press and tighten the die down. And I'm neck sizing. If you doubt it, blacken an entire case and run it into the die and see.
When the cases get hard to chamber or I'm reloading hunting ammo, I remove the washer and then I'm F/L resizing.