I've been told that you should never stand up and pedal, always stay in your seat. Are there any parts of a ride where standing can benefit you? Staying in my seat seems to conserve my energy, especialy on a tough climb.
Full suspension - stay in the seat! let the shock do it's thing and spin.
Front suspension - if the climb is rooty or rocky you might have to take your weight off the back end a bit to let the tire clear it.
In both cases standing up in the saddle is fine when you need that last burst to get you to the top. You will get more power when you stand up and be able to keep the front tire nailed to the ground. It sucks up your energy reserves like crazy though.
As for other parts of riding (meaning not climbing) getting out of the saddle helps out alot. Most people that ride hardtails get out of the saddle to sprint and get over rooty sections quickly. It saves your butt from being trashed over the bumpy parts.
On a FS most people stay seated throughout the ride. Again if you need to get weight off the bike tire sitting up a little bit is helpfull.
I guess the best advice is don't be afraid to move your body around in the saddle. Learn to adjust your body position based on the conditions so you get max power and balance. You can also shift your weight in the saddle by sliding your butt forward (keeps front tire down during climbing) or backwards (more leverage on the rear tire during downhills)