Woodsmen, I think it depends on a lot of variables.
IF the right wing view is purely political (that is to say NOT based in religion), then there may be a better chance. However, if the political stance is based in fundamentalism (usually protestant Christian) then I would say highly unlikely that the grand parents will ever change their views. In the latter case, their sense of identity and personal salvation forbids it. They *might* accept the fact that a grandchild is gay but they would never accept that homosexuality is anything other than morally and ethically wrong.
As to whether or not it is worth telling them… I wold say that is up to the grandchildren to decide. Do they want to be out to them or not.
I'll give you my case as an example. My mother and two older sisters were fundamentalists. My dad was less religious but was extremely bigoted. 'Niggers', 'Chinks', 'Spicks', 'Jews', 'Catholics'… you get the picture. Anyone 'other' was 'bad'.
So here was gay little Mike growing up in the midst of this being told over and over and over again "You're stupid, lazy, good for nothing, and will never amount to anything."
I *never* came out to these people. No way in hell was I going to do that. They were so dysfunctional and 'down' on me, criticizing me at every turn -- not even knowing my sexual proclivities -- no way in HELL was I going to give them any more ammunition to murder my self-esteem than they already had. FUCK THEM already!
If I had had better self-esteem growing up, then maybe I would have but the point is, because I didn't, I had to struggle to keep my sense of self, and keep myself from blowing my head off. Last thing I needed was their shit. So, to me, it wasn't any of their fucking business what I did or with whom.