If I recall correctly, the palm forward grip is also called the key grip and palm out grip. Refering to Al Kyte again.
Here's another article on grips by Lee Cummings who is an AAGAI/FFF Master Instructor. The AAGAI is the British version of the FFF certification. Of the thumb on top grip he writes:
"Ok, let me move onto a piece of advice I have heard over the years by various non AAPGAI instructors....
'The rod should be held comfortably in your hand by laying the rod where it meets your fingers'
Lets look at this advice in slightly more detail, remembering that the client is paying to receive and take all advice literally........
The above image is the position the newcomer to the sport (who cannot decelerate the hand sufficiently to conclude the back cast) usually ends up in. The untrained muscle "memory" of which is required to "Stop" the rod is overcome and we know through experience that this position in efficient casting is not one of which we desire. We will generally see that when the wrist has "Broken" and the rod has gone too far back, the line will be sent low behind the caster, probably after an inefficient "Non Loop" had also been formed."
See illustration and text below.
Holding a fly rod to make a fly cast by Lee Cummings AAPGAI
You don't want to change what is natural for a beginning caster if the beginner can make the proper backcast stop with the thumb on top grip. Most cannot, especially if they have been spin fishers. Then I recommend a grip change because it is important for a beginner to have early success in casting 20 - 30 feet which is easily done with with placing the rod more in line with the forearm