That's what I do with any codebase I get staffed on, at least. I don't need a coding guidelines document to determine what's expected. And if I commit any oversights or transgressions, a few code reviews should sort that out.
I understand not everyone can do this well, especially junior devs, but hopefully with it being pointed out in enough code reviews better habits with the codebase start forming.
I've worked on a couple of teams that had coding guides, and I looked at it once but never again after that, and most people didn't seem to follow it too closely anyway, everyone just seemed to coalesce around an expected code style eventually.
Maybe other people have had better experience with coding guides than I've had, I don't know.