Until you start to descend, unless you're going super huge in which case you keep it at 12 a little longer (and I like to wiggle it back and forth to slow my descent from up high there anyway). Once you're back down to 3-5m above the water start steering one way, and if you didn't get any higher than that then steer it as soon as you start descending.
That's how it works in my brain anyway.
When you say 'keep the power on' do you mean keep the bar in? Let's assume so, in which case the answer is 'yes'. More air, and softer landings, provided you steer the kite the right way at the right time.
Darren Marshall has an excellent list of 'how-to's over on his site, here is the boosting one:
kiteboardingschool.com.au/index.php?option=com_content&task=view&id=36&Itemid=78