For me, the Right Hand Rules. After all, the sound comes from the bow, not your left hand fingers pressing on a string. But intonation, which is arguably an important skill, does come from the left hand.
In my lesson yesterday, I was working on 4 measures that are particularly gruesome when played incorrectly and particularly fabulous when played correctly. Unfortunately, if you’re off on one note, it’s likely to throw everything after that off and you’re now playing in an entirely different key! Doesn’t work well if you are playing with a piano accompaniment!
My brilliant teacher sums it up with 2 words: Shifting and Spacing. Everytime you shift, your hand position changes, getting smaller as you move down the cello towards the bridge or larger as you move back toward the neck. The shift puts you in the correct place and then your hand has to change to the correct spacing for the position you are now in. When you have a small shift, 1/2 note or one note, the difference isn’t as great as if you’re shifting from first position to fourth, for example. Even greater when you’re shifting from first position to the “home base” for thumb position. And it gets progressively smaller as you continue toward the bridge.
This realization was helpful to me in thinking about how to practice. More than practicing finding notes (and I’m a great believer that there is no guessing in cello!), it’s really about practicing the shift and creating the correct spacing with my hand and fingers. The notes will be there.