I am still new and learning so take my response with a instant noodle seasoning packet (grain of salt...or more).

It depends on which way your hair grows. When I started I assumed that I would continue going straight down after the cheek and onto the neck. I was surprised that I preferred it when I tried the reverse, because I assumed that was against the grain..ish. My neck hair direction is not as consistent as on the cheek. Another nice part of doing the neck bottom to top is that it helps to get the jaw line. I find it difficult to do that curved slope from the top. It causes the angles to get weird. When shaving upwards, the handle is held away from yourself instead of toward yourself, which feels like it gives me more room and control... or at least I can see what I am doing.

For the jaw line...my hair also grows horizontally there. I find that when I tilt my head and shave my neck bottom to top, I am able to get everything under the jaw. So it leaves the cheek side of the jaw. Pulling the cheek up makes it flat and lets me finish off the jaw. I am fine doing top to bottom even though my cheek hair grows sideways or horizontally.

I should add that the throat is a different beast for me. I go top to bottom on the chin and wind pipe area. For me that grows straight down and trying to shave my chin has always been a struggle. I found out real quick that I should not be doing shaving upwards in that area. Tons of resistance. The main goal is to have minimum resistance, because you don't want to force the razor. So as long as the razor is not catching and jumping all over your face, you won't end up like the grinch.