20 lbs genesis bows for NASP are compound with no let off. I shot, and taught to shoot, and the program training is to shoot with 3 fingers under arrow. I wouldn't try it on my 72 lbs bow though. Risk of torquing string off cams. Some people shoot as you do and I don't know if it matters much, I just wasn't taught that way and didn't teach that way. You may contact the nock or inadvertently pinch it though
Edit :
Check this thread as caution to using fingers to draw compound
http://outdoorsmenforum.ca/showthread.php?t=231398