My MIL didn't come to see the kids. I mean she *said* she couldn't wait to see the kids - but then that's all she did. SEE them. As in walk by her, play in the yard, etc. I don't even recall her reading to them, telling them stories, etc. She would sit and talk to her son - to the exclusion of everyone else. They'd gossip about the people "back home" and the various family members back there, but she rarely interacted with the kids beyond simple small talk.
But she's an Ex, so I don't have to worry about her anymore!
MIL and FIL don't so much as acknowledge our kids either; actually I still haven't figured out why they visit. MIL sits and reads the whole time she is here, FIL sleeps the whole time. The only thing they actually do is offer to take us out to eat.... the offer usually is made while I'm making dinner. One time at their house about 10 years ago, they were allowed to watch my son once. He was four months old, and my husband and I left him there to run to the store less than a mile away. When we came back about five minutes later, MIL and FIL were on the porch chatting with their neighbors. My son was in their bedroom (opposite end of the house), on their high bed (high enough they have a step stool for it), with their three cats and two dogs. He wasn't even in the center of the bed and there were no pillows around him. They haven't watched my kids since.
My mother has went against my wishes for my kids enough times that she isn't allowed to watch them either. But it wasn't something as minor as rocking them. She took my son to a bar to visit her friends, couldn't understand that I didn't want my 35 lb child in the front seat of her car, chain smokes in the car without rolling down a window, has loaded guns in her house (she feels its okay since they are under the bed), etc, etc.

And she still thinks I am just being an overprotective parent and spiteful by not letting her have more of a relationship with them.
To the original poster's question: I think that where to draw the line about the parents' wishes definitely depends on the situation. And definitely rock your grandbaby, they are so sweet to snuggle with at that age.