Agree with stiebs that if it is balanced, then it doesn't matter. Just make sure you open it before nature calls, and since it does not need to be closed, I just double-check mine as I don the suit.
For unbalanced ones, you want to avoid having an air bubble inside the tubing. Easiest way to do that is to open the valve and let a little bit of pee out before descending. Orientation does not matter at all. But once you have descended more than a few feet, you're best off holding it, because it might take more pressure to relieve a kink than it does to unseat the catheter.
Whatever you do, make sure that you keep it clean. I keep a squeezable bottle (like a soft ketchup bottle) full of alcohol in my truck, and give it a quick flush after disconnecting. Some people prefer vinegar, or plain water, or other liquids. As long as you get most of the pee out the same day you use it, it'll be fine.
