Never. Not only do I NOT wish to get into trouble, it would show a complete lack of respect towards a total stranger.
There are certain boundaries in which I do not cross. And one is not hurting someone, intentionally, EVER while in this business.
If a client wanted me to do this (and I've had several clients who have wanted me to visit their house when the wives are away, and that's fine), I would think less of him and decline.
I've gone over to a man's house when the SO is away. I've not liked it very much but a lot of men like having the familiarity of me being in their house and perhaps, I'm feeding into some mental thing that he wants by having me in his bed.
But not while she is there. It crosses a line for me.
Interesting question, though. I'll be curious as to the opinions of others.