I'm comfortable with any arrangement with one rule. If Laurie says "No, I don't want to do that." and then has to repeat herself, I want to be in earshot so that a lesson on the meaning of the word "NO" can begin promptly.
If we know and trust the couple well I'm more comfortable with more distance. It's a safety thing. Nobodies going to force my sweetie to do anything she doesn't want to do.