I call my in-laws Mom and Pops and they seem to be fine with it, of course I've been around them for almost 22 years now. I started out calling them Mr. & Mrs., got more comfortable with them enough and they don't seem to mind it since I am the favorite daughter in law anyway. LOL (it's true)
As far as Dr's offices go, they call me by my first name most of the time because they are afraid of butchering my last name, which isn't that difficult to say, it's pronounced just like it's spelled.