Loosening the shoulder straps makes it easier to put the backpack on and to take it off, but you should adjust the straps immediately, so it rides high on the back.
This low back position may lead to a forward lean, which places pressure on the lower back. Loose straps also allow the pack to sway back and forth when you walk, which can be uncomfortable and may result in the straps chafing your shoulders.
This commuter is wearing his backpack low on his back, with loose straps.
Sources: Macias BR, Murthy G, Chambers H, Hargens AR. "Asymmetric loads and pain associated with backpack carrying by children." J Pediatr Orthop. 2008 Jul-Aug;28(5):512-7. Macias BR, Murthy G, Chambers H, Hargens AR. "High contact pressure beneath backpack straps of children contributes to pain." Arch Pediatr Adolesc Med. 2005 Dec;159(12):1186-7.
