Laurence Payne wrote:

What do the singers say?

If they knew this question wouldn't have come up. And in many
professional situations the producer will determine how this will be
done so that the final effect fits nicely into somebody's vision of the
final product. Singers I work with are happy to approach it anyway I ask
them to.

A problem often in less experienced situations is that nobody has a
vision of a final product. Another problem can be having the session
player direct the engineering. If it's Ry Cooder then it could make
sense to hear out his idea of tarcking himself. If it's one of my local
yokels that could lead to disaster.

--
ha