Flat is nice and easy for hand carving and works well enough. The front face can be convex or concave depending on the choice of aerofoil. If you use other than flat you will need different angles from those commonly used. Concave front faces are associated with high lift sections and may be good in low wind. Most of the larger commercial machines use convex front faces for greater structural strength with spars up the middle.

Symmetrical shapes work ok if you choose the correct pitch angle, but you will have to experiment.