I don't use any extra support when I knap but you've got to put your fingers somewhere, so it's best to place them near the point of impact.
This might cause some flakes to stop short because of the slight finger pressure but it's better than spacing your fingers too far apart and causing a "bridge" that might cause the point to snap in half.
Other knappers from a forum that I will not mention,
have been using an indirect horizontal "shaft" punch, slow strikes, and lots of support. This is fine for short and fat points but it will quickly snap a long, thin point.