In the old days, when packs were deep black holes, I packed least used items first, at the bottom of the pack, and more frequently, or first used items at the top of the pack. Weight and cg distribution were secondary.
Now, I only buy packs with panel-access, and this seemingly small design element has changed the way I pack and even the quality of my hiking experience. Now I can pack for weight and cg optimization while knowing any item is available quickly and easily. I can do things now which were inconvenient, or time consuming, or previously annoying to my fellow hikers. A few examples; on a winter hike rest stop I can pull my jetboil out of the bottom of my pack and have a hot cup of coffee in 2 minutes flat. I can secure everything inside the pack and leave the side pockets available for a shell, camera, etc. I can more easily change my clothing layers as required, removing and stuffing them neatly into the pack. I can do some of this on the move by slinging the pack in front and inverted as I walk. Also, in camp I used to spread everything out so it is accessable and visible. Now I leave it packed up, and access each item through the panel.
True, the difference in this packing/utilization method is really just the difference in time between using a panel to get your gear, or opening lid+untying and unrolling the extension collar+removing some gear to get down to what you want, then reversing that process. But done enough times, that really adds up. Some people do not access their pack much, and so this would not make much difference.
Recent panel access packs acquisitions are an REI Pinnacle, Lafuma Race28(modified), and now either a BlackDiamond Quantum or GraniteGear Meridian. I do not consider non-panel access packs larger than say 30 liters..