Google is now explicitly forbidding the following when requesting reviews in their latest terms of service update:
1. Asking for specific content in reviews. When requesting reviews, you should no longer be asking for specific services to be mentioned or anything relating to the specific contents of the review.
2. Requiring employees to reach a quota for reviews.
3. Offering incentives of any kind in exchange for a review or updating the contents of a review.
4. Pressuring customers for reviews while on-site. Ask once, and if they don't say yes, let it go.
You may still ask for the review to represent their genuine experience.