Like mentioned above I think there is no golden rule. It depends on the board and the conditions you ride.
Last weekend I had great conditions at a great spot, where you have everything, from mirror flat water, small chop, up to big chop/small waves (breaking). Wind was around 20 knots, all day long.
And so I just tryed out my directional (not a real surfboard, but "surfboard like"
) in all those conditions.
Looking for what works and what doesn't, how efficient is which riding style and foot position, better ride the board flat using the fins, or more Twintip style using the edge, and so on.
What I found out for me and my board, for going upwind is, the choppier it is, the more "Twintip like" I want to ride my board. Use the edge, put my feet closer to the rail and get my weight more on the back foot and the heels.
On the other side, the flatter the water the flatter the board.
I like to keep my board as flat as possible (but still comfortable) and put my feet closer to the centerline (back foot more or less over the fins) and then I try to "twist" my hips (and Board) a bit upwind, in other words try to "pull my front foot upwind" a bit, to get the pressure on the fins and direct the board to go upwind.
For board types, I think my board is quite easy to ride like a twintip, cause it's actually built like one and has nice rails, but if you have a board with rounder rails, edging it won't add that much to upwind ability then with sharp rails.
I have a Cabrihna Double Agent board. (without the foil
Thats just what i personally noticed. Maybe it helps somebody.