We have mandatory code reviews for all changes and I think it works quite well. A person making a one or two line bugfix may think it's quite innocuous but it may have a bigger impact on other parts of the system than they realize. Many eyes help reduce the likelihood of that.
I dunno... the odd bug fix has the potential to really screw things up, when the person fixing it isn't the person who wrote it originally, or it's been months since then.
However, I don't see as much value in reviewing low level changes such as one liners or odd bug fixes.
Making code reviews mandatory is too much overhead and too inflexible for me.