Why is it insecure to think that there are class connotations in the names that are chosen for grandparents? There are both regional and class variations, which is probably one of the reasons why there are so many variations that can be used.
Here's a sociological paper about it all: books.google.ca/books?id=YBOqD78x9zkC&pg=PA256&lpg=PA256&dq=class+implications+Grandma&source=bl&ots =kAcQpbshAM&sig=03xR2qrBO82cU2Pp23WlbpnUOdY&hl=en&ei=1GjRSojvEoH2sQPUw4TwCw&sa=X&oi=book_result&ct=r esult&resnum=1&ved=0CA4Q6AEwAA#v=onepage&q=class%20implications%20Grandma&f=false
Apparently Mum and Dad are working class terms in origin (with middle class children moving from Mummy/Daddy to Mother/Father). I don't know anyone who calls their parents Mother and Father though (you do see this in older films though), plus the regional variations are often different from the class ones, so something that might be posh in one area might be working class in another.
My view is that the person who is called by the name in question should decide on it. IME most grandparents do have quite strong views about what they would like to be called, for all sorts of reasons. My mother for example couldn't settle with any grandmotherish name, which I think is because she felt ambiguous about becoming a grandparent (the occasion wasn't terribly auspicious in her eyes).
For the OP, I really don't understand why you want to call your MIL Grandma. Unless your children are the first grandchildren it's not really up for debate, as the name has been set. I don't know any grandparents that are called different things by their grandchildren - even ones like my maternal grandparents, who got slightly odd grandparent names when my big sister (first grandchild) decided on something a little different, all the subsequent grandchildren called them the names (obviously after my grandparents adopted the names).