I think its all about what kind of relationship you have with your "step-mother"
My dad married my stepmom when I was 5.... she was and will always be "Leanne", not mom.
On the other hand my mom married my step dad when I was 14, and I call him Dad.
If you don't feel comfortable calling her "mom", then don't. Maybe you will be able to call her mom in time, as you grew closer, but then again maybe not.
I'm closer to my step-mother now, as an adult, and I know she thinks of me as a daughter, but it just doesn't feel right calling her "mom"