IMO, it is a matter of personal preference, but I heel the straight grip or dish on a traditional longbow, use less heel on an R&D longbow, and none on the newer style recurve. The old 1955 Bear - 58 have a straighter grip, but I would use the pad of the thumb, and not heel them. The raised palm and slope on a recurve puts the pressure on the thumb and web in your hand. The back of my bow hand is almost at a 45 deg. angle to the plane of the bow. On a Longbow the lifeline is where the pressure is located and less on the thumb, the back of the hand is vertical, or in the same plane as the bow. This is why it is difficult for many to be able to shoot both styles of bows. G. Fred Asbell talks about it in his latest book.
When Howard Hill heeled the bow, the pressure was along the lifeline, hence the heel of the palm.