Hi Brendon,
Ron's advice is right on, any good skier can ski powder on thinner skis. The only exception is trying to use a serious race or race-carver ski in deep snow, these are useless in bottomless conditions and will dive like a u-boat.
Skiing in deep snow on a thinner ski requires good technique and is much more difficult for heavier skiers than for lighter skiers. see the "Can a Head Monster 82 float as well as a Volant Spatula?" thread on this forum for more info.
But it really does come down to preference. If I owned one ski and skied the groomed runs 75% of the time I spend on the slopes, I would stay at or below 69mm wide underfoot. The wider skis are more versatile but are not as responsive to carving technique. A ski like the Fischer RX8 or Head Supershape will carve as well as any "skicross" type ski and still allow good skiing in moderately deep snow if the skier is skilled. It helps if the skier is also lighter than average, too.
Cheers,
Michael