I wear my pack VERY differently than most because it would produce too much heat if I didn’t. I cinch up the hipbelt and let the shoulder straps stay loose, so the pack doesn’t lay on my back. There may be an inch or two between my pack and my back, so the air can circulate and I don’t get sweat rolling down my back.
Most people say you loose control of the pack if it’s not worn properly, but I am either used to it enough to compensate for control, or they are wrong because I don’t have a problem with loss of control.
I bought a pack in the middle of a thru hike in '02, trying to save a couple pounds, and started using it when I got to MA. The new pack was built so it HAD to lay tight to my back. It felt like I was wearing a heating pad. I made quick arrangements and had my old pack sent back to me and was happy again.
The weight you carry would make some difference on control, but my pack in '02 was 39 pounds when I left Amicalola Falls. It averages about 20 pounds now for most of the three seasons and 26 pounds in winter. I wore that pack filled with 39 pounds of stuff loose in the shoulder straps too and had no problems.
I’ve been using ULA packs since the beginning of '04 and like them a lot. 
Stumpknocker