When I busk, I don't want people to give me money because they feel they somehow should - I want them to give me money because they like what they hear enough to encourage it.

So, I use the same rules for whether I give money to buskers. If I like what I hear, and I'm not in a hurry to get somewhere, I will give them some money and maybe stay and listen for a bit if it's convenient. If I don't like what I hear I ignore them.