At Starred, we do not believe in mandatory answers. Sometimes you just don't know an answer, you have no experience with the issue or there is something else why you can't give an objective answer. That's okay, and the reason why you can leave some questions open, as feedback-giver.
How does this work for you, as feedback-asker? Go to your dashboard and select the form of which you want more information. When you move your mouse over the results, you see how many respondents answered the question. This can help you discover which question is less often answered, and maybe need some adjustment.