As an inexperienced flyer....
Why is palm up, or near palm up so bad? It seems a good way to me to differentiate control when inverted. I've watched videos of some very good fliers using this technique.
I have yet to fly inverted. In other indeavers I've learned muscle memory, while great in many areas/times, can be disastrous in others.
It might help if you are just going to fly inverted, but it can be absolutely catastrophic doing more complex maneuvers. My example above was very revealing, the gentleman in question had to try to twist his arm around all over the place to make his hand upright or flat, depending on where he was in the square 8. This made/makes any attempt at precision hopeless.
If you are only ever going to sport fly, and not compete, then, it doesn't matter too much. But if harbor notions of stunt competition, I strongly suggest that posture and handle setup is something that can, if you get too far off early in your career, you are going to have serious issues and make it all much harder on yourself. I have many examples of that, and several examples of "doing it right" from day one - including a sterling of someone who *never learned to do it wrong* going from not being able to fly inverted to being competitive in Advanced class stunt in 3-4 practice sessions.
So, you learn something that helps and builds in bad habits to get through the first few hours without crashing, that then sets you up for a lifetime of fighting it. It's a classic example of people making something intrinsically simple into a complex mess.
As an aside, the same thing applies to the engine and airplane as well, particularly trying to use vintage *engines* and trim ideas, and then spending months/years struggling.
Brett