Interesting that this seems to be a new idea for many folks. All the coaches I've worked with for the past several years (Jonathan Ballou, Reilly McGlashan, Tom Gelle, Josh Fogg, JF Beaulieu) advocate a dorsiflexed foot at all times during skiing. By spreading and lifting the toes, and trying to feel that pull/lift all the way back to the ball of the foot (i.e., not just in the toes), you create a functional tension that plants you in the strongest part of the foot (back of the arch, front of the heel) and activates the ligaments of the front of the shin to automatically pull you ahead. Try it.
An exercise we do with students to demonstrate this is to have the student stand on flat ground in his/her normal stance on skis. If we kneel down in front of the skier and push/pull the tips about 8 inches back and forth, the skier will invariably be very unbalanced. Ask them to dorsiflex their feet in the manner described above, and they are much more planted on the skis and their center of mass moves with the skis in a secure and balanced manner. Fun exercise to try!
Obviously there are several schools of thought about dorsiflexing, plantarflexing, and the most functional place for the weight to be on the bottoms of your feet. Try them all all and see what works for you!
Best!
Mike