I believe that you have to do what you feel is the right thing for you - first and foremost.
Secondly, I believe that we attend funerals to show support and comfort for survivors. If your spouse feels very alone at a this time, it may be a great gesture to attend if for no other reason to support her.
If you feel that you want to honor the grandmother in another (more personal) manner, maybe organize that. Keep yourself occupied getting ready for the real memorial of her life that can be held for her closest friends following the cold, impersonal informercial at the Kingdom Hall.
I feel for you. And for your wife. I hope Grandma is enjoying herself. Somehow 90 just doesn't seem as old as it used to...
-Aude.