If you're talking about percussion knapping, then a thicker spall is easier to work and get good long thinning flakes from. Percussion doesn't work too well on thin stuff. If you want to pressure flake arrowheads, then as everybody said, thin flakes are better to start with, but you'll have to shear the thin edges back enough to get a good platform to flake from. The goal of percussion knapping is usually to thin and shape the piece down to the point where you can pressure flake it. Also depends on how big of points you want to make.