Indeed, nothing is mandatory! It's your opportunity to thanks a guest, give other hosts a heads up if a guest did not follow your house rules, notify Airbnb of something in a prvate manner, etc.
If you ignore the prompts to leave a review, there will be no way do do it after 14 days. If one of parties leaves a review and the other one doesn't, after the 14 day period that one review will automatically be made public!