The short answer is that it usually doesn't matter. If you are using a thru-neck design as is common on CBGs, then the heel usually doesn't add any needed strength. On a traditional guitar, the neck does not extend through the "box" so the heel provides some rigidity for the neck joint at the box. On a thru-neck, this does not apply.
Since I like the look af a heel and usually build a stepped neck inside the box, it is easy create a heel even if it is only decorative. I usually have the heel extend about two inches out of the box.
This a clear case of the CBG motto: "There are no rules." Decide what looks good to your eye and go for it. You will not be wrong no matter what you decide.