When I played cash games, I always used auto top-up. As others have said, if you think, you are a winning player, then why not give yourself the best chance to win a big pot? Its also about not complicating the game to much by constantly having to adjust to different stack sizes, when you lose or win a hand. Sure that might happen, because other people play with weird stack sizes, but no need adding to it by doing so yourself.
For the same reason I did sometimes do, what it known as "ratholing", when I played Zoom. If my stack had grown above 200BB, then I left the table and opened another entry to be right back at 100BB. This was because, I felt, I did not have an edge against the field in 200+ BB poker, and I also did not need the extra variance.
If I want to play 40BB poker though, then my choise will always be to play tournaments and not cash games. So if you ask me I would be completely happy, if
poker sites simply fixed cash game buyin at 100BB, nothing more and also nothing less