[Murray]

First off, customise away. Everyone's hand is different, and if you don't customise, how will you know what works?

The primary indicator of poor grip would be the bow leaving the bowhand at an angle, or twisting on release.

However, check your bowhand tension as well as position. In particular, too much tension on the thumb side is, in my experience, the cause of a significant amount of horizontal group spread.

Try shooting short range without a stabiliser and see what happens to the bow (best with a buddy watching your bow reaction, or with the assistance of a video camera).