It's purely user preference.
there are right ways to hold the mouse for certain movements
That's what would make the most sense.
While many things in life are up to choice, I wholeheartedly believe that nothing is "preference", rather there's a right way of doing things.
You can choose to do or dedicate yourself to something, in the end though, how it's done correctly is not up to you.
Regarding mice I could never find such a discussion on the web, and wanted to start one myself, but lacked the motivation, because it would be a lenghty one, and required knowledge that IMO isn't even properly researched yet.
Would you suggest someone who wants to learn to play the piano or tennis to press a key or grip the racket however one sees fit or thinks is the most comfortable way? Surely not. Over the years scholars, and trainers alike have found, and established ways how to properly learn to play an instrument, or a certain sport to not only ensure progress, but also healthy practice. It's what you call the fundamentals, or "textbook". Whether you strive to win Wimbledon, or just want to play with friends every other weekend, the fundamentals are the same, and everybody has to learn them.
Now, I believe, it's the same for gaming, especially in how you hold a mouse, to not only progress in the most efficient way, but stay healthy while practicing. E-sports is rather young, and while there are fundamentals for top level competition, it's noticeable that those fundamentals are only regarding a game's mechanics. Be that to know hard, and soft counters, or build orders for RTS, or timing items, and knowing the most efficient routes through maps in FPS to name just the most basic fundamentals.
However, much like with playing the piano or tennis, that's just the cognitive part. With all those activities there's a large amount of dexterity involved. Without the required dexterity you'll have a hard time executing all those cognitive fundamentals.
Off to bed, more of this later.