I don't allow it. I cannot give the children my full attention (which I am being paid for) if I am also monitoring a visiting parent. Current clients are not paying for me to play hostess and allow strangers around their children.
NEVER do children behave normally around strangers and any provider is going to act like Mary Poppins when being watched, so it doesn't ever give a parent a true look into what a day at the daycare is truly like.
I get goofy with my kids and will not ever do that being watched by an adult I do not know!!!
I tell parents to check out my references (I include both past and present clients) and to check my licensing record. Other than that, I expect them to trust me or look elsewhere for care.