When using a fixed crawl, the objective is to be able to hit your intended target by putting the arrow point on the spot you want to hit (or slightly above or below depending on distance). I think changes in cant could cause the arrows to miss from side to side. The fixed crawl is a variation of gap shooting. Every gap shooter I know either keeps his bow vertical or alternatively, has a small cant of maybe 2* that he consistently maintains from shot to shot.
I suppose it would be possible to experiment and find out what effect different cant angles have on where the arrow impacts, but the whole purpose of using a fixed crawl is to simplify the aiming process, and here we go complicating it again.