Late to the conversation like usual, more on that later.
Hand position is only important IF it is causing you trouble (hand pain, limit in speed or dexterity…etc).
In a perfect world, with perfect finger length, perfect posture, perfectly fitting guitar neck, and perfect strength and balance (finger to finger), having your thumb behind the neck at a point directly behind the space between your middle and ring fingers and not going over the top of the neck of the guitar will give you benefits over most other hand positions. This position will allow you to reach 12 frets with your fingers without moving your thumb. It also is a very relaxed position for the muscles in your hand and wrist.
BUT
it is not the end all be all of hand positions for all guitar styles on all guitars with all guitar players. And the fact is that many if not most great guitarists do not adhere to the above position for their left hand.
My point is that unless you are having pain or are finding certain techniques impossible to play unless you use that hand position, you do not need to reinvent the wheel.
Update: Now to why I am not here as often as I would like. I recently retired from skilled care work and a retirement community. The next day, I started to clean and reorganize my home. You see I am going back to instrument repair and teaching in my studio and the last 7 years has not been kind to my wife’s, son’s and my storing habits. It took a month of putting down subfloor in the attic, organizing every bit of storage in the house (keep, yard sale or trash), properly storing it and moving to the next room. The entire house is back to some semblance or good order. Once that was done, I renovated my kitchen with new plumbing, additional electric, refinished cabinets (26 cabinet doors, 11 drawers and all the frames). In addition, I needed to build 2 additional cabinets to match the others. So the cabinets are all finished and hung, the new granite countertop is installed with all new sink and fixtures and the walls have been painted (a very unusual warm gray/brown (my wife picks the colors). I still have to add a tile back splash and install the floor. On top of it all, I still have a torn tendon in the back of my left hand so I have not played the guitar for over 2.5 months
The plan is that once I get back to teaching, I will be working 3 days a week and I will have some time to do the things I enjoy (like play the guitar ) I miss visiting here and hope time will allow me to come and learn and share again. Take care all!