I'm coming at this from a couple of different perspectives, so bear with me :) While my FIL is remarried, we don't have our kids call his current wife grandma, there are some weird circumstances that make us not comfortable with it. Althouhg, his previous wife was known as grandma to our kids, we never saw them much and she is out of the picture now.
On the other hand, my brother is married, his wife has children from her previous marriage. We consider her kids part of the collective grandchildren group, my parents treat them like the rest of their grandkids. I haven't heard that there are any issues with that or that anyone is upset that they are called grandma and grandpa.
BMs, how do you feel about your children getting a new set of grandparents? Do your parents feel threatened at all?
SMs, how do your parents think of your stepkids? Do they consider themselves to be their grandparents?