I think it depends on the type of speed your trying to attain and the style of kick. For me when doing just a general dive of exploring, I go very slow and usually do small kicks with knees already bent up. This type of kick gives more power on the down stroke. The same goes for a modified traditional flutter Kick, where I keep my knees still and bent up slightly and kick never fully straitening my leg. Another reason for these type of kicks is that it allows me to get closer to the bottom without disturbing the silt and ruining the viz.
Even when I do a full on traditional flutter I get more power on the down stroke. I think this is because I have a tendency to keep my head lower then my feet again to keep from disturbing silt.