I think it depends on the relationship.
I was very close with my ex-husbands family. When I was with them, I felt like part of the family. I called his parents Mom & Dad. I even referred to his sister as my sister (never bothered with the -inlaw part) & his niece was my niece. It felt right & comfortable for all of us.
I had an friend who also had parents that I called Mom & DaddyBob. But then again, that is just how that family was. ALL of his friends & his sisters friends called his parents that. None of us EVER thought to call them anything else.
My Mom was never offended because she understood that these two families welcomed me with open arms (& hearts) & treated me the way family should be treated. They all had my best interests at heart & respected me. Both of these were homes that when I walked into, I immediately felt comfortable & at home in.
My current husband - well I am not as close with his parents. Either I don't call them anything, or I once in a while refer to them by their first names or Grandma & Grandpa. They do not come across as loving or as welcoming or as thoughtful as the other families. Actually, they come across kind of coldly. If I ever even THOUGHT about calling his parents Mom & Dad - my parents would be crushed!
If/When I get a son in law (DD is only 8, so I have a while), if we feel close, I would hope he calls me Mom. Otherwise, I would want to be called by my first name. Mrs.XXXXX is my MIL & I am NOT her (thank God!!!!)
My cousin solved the problem creatively.

They were married for like 10 years before they had kids & could call them Grandma or Grandpa. So she called her M-I-L & F-I-L: Mil & Fil (like Mill & Phil). I got the biggest kick out of that!! It works for them. Personally - I wouldn't want us to be called that, but I still love their solution.
