When I'm talking with them there mama and daddy. When we were still dating I called them by there first names. but after a year or so, and they helped me recover from a major surgery it just felt right. I could never call her 'mom' tho, mine passed away 8 years ago and that just hits to close to 'replacing her' and a few of my close friends' moms have always been mama 'name' mama 'lastname'.
As for 'daddy', it's just something his mom got me started on, they have fur-babies and when she talks to there pug she's tells him 'aw, don't worry buddy, daddy will be right back' and it just kind of stuck i think it makes him happy tho they've told me I'm the daughter they never had. so it makes them feel more like were family.
Although it does cause some confusion around the house with DH, I think he forgets I was the baby at home, and still sometimes call my dad that when I'm on the phone with him (we're very close I call him everyday/ text him though out) but he has to ask which mine or his.