The way you load your hiking pack should be dictated by the type of hike at hand. If you're going on an overnight (or multiple nights) treks, then you're going to have sleeping gear and shelter of sorts, and these items will only need to be accessed once a day, thus they can be loaded into the bottom of the bag.
If you anticipate rain, keep your poncho or other foul weather gear easy to grab, as getting wet is both unpleasant and radically increases the risk for hypothermia in all seasons. Water purification systems and first aid gear should always be readily available. That, of course, goes for your water supply itself, too, and your camera.
But beyond those few items (water, emergency gear, e.g.) that you need to access readily and at times quickly, the most important factor when it comes to packing is weight distribution, not accessibility. You need to keep the bulk of your heavy gear as close as possible to the small of your back. That helps to keep it near your center of gravity, which means better balance. Better balance, in turn, means more safety and less fatigue, both crucial factors when you're out in the wilderness.
If you're hauling lots of extra water, go ahead and load it low in the bag. If you have lots of clothing for layering purposes (or simply because you're out in the cold) get that up near the top of your load, as it's lighter weight. And be sure to give your pack a few good shakes and bounces once you have loaded, and then check its weight distribution again: things shift around as you move, and even a pack that seems properly loaded can quickly become off balance. Take the time to readjust or even repack during breaks; your back and the rest of your body will thank you.