These vary from city to city. If you are familiar with a city and would like to help improve this resource for other buskers, please write your wisdom into the comment section and I will incorporate it into the website.
Boston: Overall, Boston is a wonderful city to street perform in. The police are generally accepting of you as long as you are considerate and not blocking the side walk too much. I have never been asked to pack up. Just use your common sense. Here are some good places to play:
Harvard Square: Permit Required, click here for information about it.
Boston T Stations: Permit Required, click here to download the .pdf application.