Google is now being very explicit about this in their guidelines, so be careful.
New policy. The new line that was added to the contributed guidelines is “discouraging or prohibiting negative reviews, or selectively soliciting positive reviews from customers.”
Is it new? Most local SEOs say this line is not actually so new, that they all knew that this was not allowed but now Google is making it crystal clear. Colan Nielsen said on Twitter that this is “not really new information but worth noting since Google is being more explicit about it.”
Why we care. Just be careful how you solicit or discourage customers around leaving or not leaving reviews for your local business(es). If you cross the line, it can lead to reviews being removed or even worse, a business ultimately being suspended from Google Search.