Well, climbing out of the saddle uses more energy, but on a fixed gear i think it's more important to keep a higher cadence, even if you have to use a little extra energy and breath harder because you're out of the saddle. When the hill levels out, sit down, catch your breath, then stand up again when the hill steepens. If you've got gears, by all means have at 39-24 and spin at 100 rpm on your ass. Otherwise, keep that cadence high with some manly out of saddle spinning.