Our guest room is becoming a nursery/guest room. My parents stay at a hotel when they visit (mom couldn't handle the stairs to the room anyhow), and his mother doesn't visit. We're basically just keeping the full size bed that's already there because we hate to get rid of it, only to buy a new bed once we're past the crib stage.
I think as PPs have said, you shouldn't worry about having a guest bed for G-Ma's unless they're staying frequently/for long stretches, or you have the space for it.