I kind of hate unwritten rules. The self-policing that everybody is just supposed to know gives people a really good excuse to look down their noses at people. If it's a good rule then it should be written down. Begging people for retweets on Twitter is one of those unwritten rules. People usually don't do it because they know it can annoy others or because it can bring unwanted negative attention.
But there can be good reasons for doing it. Let's say that your favorite hockey team is holding something you'd like hostage to the demand for an unreasonable amount of attention, essentially FORCING you to beg for those RTs to even get close to your goal?