........... It doesn't really matter if the heel pad is directly ON the the handle or not. If not actually on the handle, we can still determine that it is
lined up with a specific corner or bevel of the handle. Here is Will Hamilton talking about Federer's "Eastern" FH grip:
youtube.com/watch?v=aXcsblS3Jl4&t=65
Will's description is not really at odds with the heel pad position he is showing -- even tho' Roger's heel pad is off the racket for the most part. Second point: Below we can see the Roger is using a slightly shorter grip for this FH volley. A little bit (more) of the heel pad is on the handle for this shorter grip.
I vote for describing things as accurately as possible with words, illustrations and videos.
At 1:49 in this 2007 video, Will describes the two references points, index knuckle and heel pad, with clear illustrations and uses the word "rests".
Does the exact location have any significance? Yes.
1) If I hold a racket and place bevel #2 on the index knuckle and fat pad, it produces a forearm to racket angle with a relaxed neutral wrist. Try it with a racket.
2) If I hold a racket and place bevel #2 on the index knuckle and have the racket butt at about the location of the little finger, with the finger gripping the handle there, the racket has a different angle to the hand and therefore the forearm to racket angle is different.
These angles may vary a bit based on racket handle and hand dimensions. ?
Try placing the racket in both positions and looking at the racket handle's angle in the hand. Try looking at your open hand and drawing the two lines across your palm. The line to where the little finger wraps around the racket takes a little estimating. Maybe chalk dust on a bevel of the racket handle would show us.
The forearm to racket angle matters for the serve and I assume for all other strokes. Especially for the kick serve the forearm to racket angle is smaller at impact than for the slice or flat serves.
Exactly what is being done with the high level strokes is not clear. That's why looking at the best performers helps understand the strokes.
Back to the OP question - Do most current high level one hand grips for the forehand, one hand backhand and the serve have the racket butt end very close to the little finger or does the bevel rest on the fat pad?