I used to be a mediocre guitar player but i moved on to bass 2 years ago. So ive been playing bass for about 2 years now. But my teacher told me that my left thumb should always be on the middle of the neck. But when I play, my thumb goes up nearly to the side dots. Also, he told me that my thumb should point up vertically to the frets but my thumb slants to the left when I play. But when I was learning guitar, my teacher did not tell me anything about it. Is this is problem that needs to be solved? or is it just a matter of preference?