Depends on how deep the powder is and how much your nose will dive down if you leave it centered. The short answer is there is no harm in setting the bindings back and in theory it helps keep your legs fresh from having to really lean on your back leg.
Sort of true. But the reference setback (or lack thereof) is there for a reason: It properly centers/positions the rider in relation to the sidecut (and in some cases board flex etc.) in order to ride the board the way it was designed.
However, sometimes conditions or rider preferences mean it makes sense to deviate from the 'design' setting (for instance, adding/increasing setback for pow riding, especially if the board was not designed for it). Just be aware that it will affect things, especially for turns - turn initiations, edge engagement throughout the turn, etc.
As long as you do not overdo it (say, more than 1/2") you should be perfectly fine.