I agree with Indinana in that it depends on your relationship.
I have two g'dtrs through my son, but my dtr and her partner have no children, and I should say that they both knew that I was certainly wasn't craving any more grandchildren, so no pressure at all from me. My daughter had always said that if it happened ok, if not, that was ok too, and she has never been off birth control. However, they have never had any spare money so, just before they got to 40 I did ask, making it clear that my only reason for doing so was to offer some financial support if that was what was stopping them. As I suspected they are very happy as they are and content being a fabulous Aunt and Uncle. No one was offended that I asked. But, I must stress, that they were already very clear that I wasn't expecting either of my children to 'give me grandchildren' - that, I believe, must always be their own choice with no pressure from anyone else. (Do totally love, adore and would die for the granddaughters I've got though.)
To add to that, I am proud of them for making this choice. Most of their friends and siblings have children and they have seen first hand that it is no easy job to raise children these days. They know that parenthood is not for them and have resisted the barbed comments and pitying looks they sometimes get.
My son and his wife have two dtrs, but had they not had children I wouldn't have felt able to raise the subject with them. We have a good relationship, but not quite close enough for something so personal.