Never. I only do first-time interviews after hours. It is just too much to juggle, and I really don't allow "strangers" (outside of some close family and friends) in to the house during dayhome time. And knowing kids, that would be exactly the time that little Sally has a total crying melt-down, and little Jimmy decides to pop little Joey in the nose!!
If we have done a full interview, and they are interested, then I may allow them to come by during free-play time (as opposed to nap time, meal time, craft time, etc). I would like to see their child interact with others as well.