When I got my 1952 Gibson J-45 (about 16 years ago) I saw that it had a strap button right in the middle of the flattest part of the heel and I thought "Oh shoot, this thing will tend to tip over...". It never did, that guitar is as balanced with the strap button in that place as any Martin with the button on the heel's treble side. So I'd suggest to take advantage of the flat heel, provided you install the button as close as possible to the heel's curve (= toward the guitar's front).