Out-of-the-saddle technique

When you're pedalling out-of-the-saddle/off-the-saddle, should you lean the bike side-to-side, or try to maintain the bike in a vertical position? Which is more efficient?

When climbing you should not really let the bike move about excessively. Most climbs will be about efficient pedaling over the distance rather than getting raw power down for a short time.
For sprinting on the track, and on the road to a certain extent, when applying lots of power it's beneficial to move the bike from side to side whilst keeping the head and body central; the bike goes the opposite way to the leg which is applying the stroke (push down with right leg, bike goes left)