I think it's your call, but I don't see the big deal. If you aren't comfortable with them singing, then I think having the DJ acknowledge it is a great choice. I don't think singing would detract from your wedding celebration at all; however, I hate being sung too, so think about if the person would appreciate it or not. If they won't (like I would be super embarrassed if someone did that to me), then all this worry is for nothing. If they would like it, I really don't think it would harm the feeling of the wedding being about you.