Should I always tell people exactly what I think?

Not exactly. Not lying, telling the truth in every circumstance, that is S-tier, and it's part of being a person of integrity. But being a truth-teller who shares everything is more like B-tier. We should cultivate saying honestly what we think, but we don't have to go out of our way to share everything we think with everybody. Some people are truthful in a way that harms the people around them: they correct everybody and everybody needs to know their opinion.

The reality is we don't always see everything clearly, and we don't want to be constantly causing disharmony because we think we need to tell the truth on every little thing. Speak truthfully and honestly, but you can mitigate your responses and not overshare.