My parents live 30 minutes away and pick the kids up from school 2 days a week, do all my school holiday cover, if we ever need babysitters we have to ask them and they do loads for us. They love it and wouldn't change it.
My PILs live hundreds of miles away so obviously aren't in a position to help on a regular basis with our kids. In 10 years they've done 2 pick ups for us from weddings on my side of the family and always tied it in with a visit to see us. We have never asked for any other help looking after the kids. When they visit we do things all together, maybe once a year they will offer to take the kids on a day out without us.
We have an upcoming family event so have invited the wider family and quite a few people are coming to stay, including my PILs. Unfortunately the start of the event clashes with a commitment for DD and she really doesn't want to miss it. OH and I need to be at the event so OH suggested "I'm sure we can get a grandparent to pick up." (It's 20 minutes in the opposite direction to parents house) I confirmed DD's attendance on this basis without clarifying which grandparents he meant - I just assumed he meant his side as they will be visiting. When I went to clarify after replying he was horrified and just assumed my parents would cover it and we can't possibly ask one or both of his parents to drive to an unknown (to them) location to help us out.
For me, we should at least ask them first as we really need the help, they are already in the area so it's not like we're asking them to come specially and they don't do almost anything for or with their grandkids the rest of the year. Only if they refuse do I feel it is fair to even contemplate asking my parents.