I would view it as a more long term low level stress if you ride with a backpack with a rounded back position.Many racers over time actually experience low back problems from the force generated pulling up on the handlebars with the excessively rounded back position common for them.You will probably just notice a lack of comfort which seems to be a common complaint of road bikers with backpacks in which case just put it on the rack.Also riding more upright or standing and stretching/flexing the back ocassionally will help if you do ride with a backpack on your back.

If you are talking about a fanny pack I find them more uncomfortable to my hips than lower back both on or off the bike.I do wear a light fanny pack for valuables etc but not gear.I used to have one that held a water bottle and it was alright but I personally wouldn't go any heavier than that.But you are somewhat younger and probably more flexible than me so the important thing is to listen to your body and just put the weight on the rack if it starts feeling uncomfortable.

P.S. The backpack looks like a good blend of light weight plus durability so well suited for a bike tour imo.