When do you think is the best timing for a solo vocalist during a wedding ceremony?

I have asked a good friend (clically-trained singer) to sing at my ceremony, and she's wonderfully agreed to do so. My question is, what is the right timing? Is it best to have her sing before my fiance and the pastor go to the alter (as pre-ceremony entertainment), before the processional while we're waiting in the wings, or after the processional but before the vows?