I'm very similar to Epacris in my approach. My DS has had one night sleepovers with both sets of grandparents since he was about 3 months. Now he probably averages once a month at either house. For us it's not about babysitting but developing a close relationship.

He is totally the boss at both of their houses and co sleeps to his hearts content.

Once he was away for 2 nights at my parents place as we had a wedding away. It was less disruptive for him to stay the extra night rather than get him up early to drop him off.

Not sure about them going away from their homes with him.... We'd have to take it on a case by case basis but he'd probably need to be a little older yet. That being said, I can't see either sets of parents asking something like that. If they went away we'd probably go too.