How do you go about constructive criticism of code?

Three simple ways to phrase these things is:

• As a question: What if we ...? How about changing this to ...? Might X also work here?
• As a further improvement: This approach will work even better if we ... We could also do X here, so that
• Say it from the point of the team: all the use of 'we' in the previous items

Combining the three generally works best in my own experience.

There's a camp that you should not give unsolicited criticism. But sometimes problems that go unchecked can end up being your responsibility. How do you reconcile?