This is really down to opinion, but my feelings on it are, you should add comments for clarity.

If you're doing something really complex in T-SQL, tell us what it is. If there's a funky business requirement, let us know that. Clarify the code. That's all. I think it's fair to assume at least a fundamental level of knowledge of both the business and the code. But if you think it's possible that someone is going to be scratching their head, help them out.