There is a big difference in the way id pack a pannier verses a backpack/messenger bag. It depends how stiff/much padding is on the back of your messenger bag. I like a straight contact point against my back if im using one of my chrome backpacks or messenger bag. I like to put softer items like shirts/pants and extra layers like shells/knee/arm warmers/etc against the back, then lunch and tools in front of that making sure theres not going to be anything poking me in the back.

If im using panniers that packing isnt as big of a deal. I usually try to keep heavier items at the bottom and obviously try to keep the weight evenly distributed.