I disagree with allowing parents to drop in anytime. When choosing a provider the parents must do their due diligence ensuring they are placing their child in a safe and enriching environment. Ask for references and do call each parent - visit a few times, at different times (only for a short period) just to get the feel for the provider and the atmosphere he/she provides - offer to bring lunch and then eat it there with the group - make sure the provider has police check, child cpr, has smoke/carbon monoxide detectors/fire extinquishers installed and available - are book shelves secured to the walls - check out the childrens nap room- have a visit to meet the family etc.
I don't mind when a parent calls to check how their child is doing in transition or if feeling unwell. I just don't want the flow of my daycare interrupted by visiting parents. It undermines my influence with the children and is disruptive. A parent can tell if a provider is serious about the quality of their daycare they run or if it is a fly by the seat of your pants provider. Trust your gut instinct.