Drop-in's drive me crazy. I would prefer a call first. Sometimes my house is not in the neatest condition, sometimes I have more dishes in the sink than just a cup or bowl, sometimes I just don't feel like dealing with people and being a good hostess.
I try to call and give somebody a heads up and let them know if I would like to stop in to say hi, that way if it isn't convienent for visitors they can say so. Even placing a phone call to just chit chat I will try and ask if it is a good time.