Alex and Ned both stay over at my parent's house every Wednesday night, and have done so since January when Ned was four months old. However, my parents see the boys at least three-four times a week, and the boys are VERY comfortable with them. My parents also literally live in the next street, about 300m away, so logistically it's very very easy.

My PIL, on the other hand, have seen the boys five times since Ned was born, once for 15 mins at the hospital, for 2hrs at Christmas, 2hrs at Easter, 2hrs at Alex's birthday party, and for 30 mins the other Saturday afternoon to drop off some presents from interstate rellies. So there's absolutely no way I would even consider leaving either of them at PILs without being there myself. (Dh himself is a question for another time...)

It's definitely more about what YOU are comfortable with. I trust my parents absolutely, and know that they have my boys' best interests at heart.