It's hard to decide which are the critical changes. Even a single line change can have major consequences - where do you draw the line...

I guess both 100% code review coverage and 100% test coverage are extremes that one shouldn't worry too much about. But my suspicion is that 100% test coverage can do more harm than reviewing of each commit.

Obviously depends on a project and team size. In a reasonably sized team a team lead would be the one to draw the line.