Why not just assume they're big boys and girls and will applaud if they choose to, and not applaud if they choose not to? If I'm in a bookstore for a performance, I'll applaud the numbers. If I'm in a bookstore looking for a book and there happens to be a performer there, I'll only applaud if I really like the song(s). If the singer said, "you don't have to applaud if you don't want to" I would take it as being condescending/patronizing.

Maybe I'm weird.