Any app that nags me for a rating gets one - a single star for nagging me!
As a developer, I resent this mentality.
There are so many apps competing to be the best, and if you have the best, the only way for others to know it is for people to actually rate and review the app to inform potential buyers.
I mean, it's fair to be upset whenever it throws notifications to rate at you all the time but if it is once or twice, I don't see it as a big deal, especially if the app is an application that you use and enjoy. It takes less than three minutes and helps out a lot - People really only write reviews whenever they are upset about a product and it's hard to motivate people into doing something whenever everything is going perfectly. It's just how people are.