I don't really feel strongly about it, though sometimes Flow takes 10+ seconds to re-check a large project I work on after changes, and I wouldn't want building to have to wait on that. (I've never worked on a similarly-sized project in TypeScript, but I expect it's about the same situation.) I usually use a CI system (CircleCI) which sends me an email if any of my commits don't pass type checking or unit tests, and it flags any PRs on Github that fail too.