John Lucas
I would say it depends on the wood and how much it warps. I've only done a few that were right at 20" and left them about 1 1/2 and they were OK, could have maybe gone thinner. They were maple and Box Elder neither wood actually warps all that much when cut as a side grain bowl.
Wall thickness when finished was probably about 5/8" but like you I don't worry about it, I just do what looks right and feels right.