It looks like you've answered your own question. It does depend on whether or not the grandparents feel they are being exploited, for they really have no obligation. They've had their turn of looking after their own children, now it is their children's primary responsibility to look after their own kids.
If grandparents enjoy occassionally looking after their grandchildren - and that would probably be for a short period of a two or three hours, after that it could become work - then fine. If they are asked to look after them on a regular basis and those grandparents could use the money then pay them. And pay them a proper wage not some token amount 'because they're family'
Of course, if it is just the ocassional favour it would be insulting to even mention any monetary recompense. An invite to dinner or somesuch should well and truly cover that.
This is, of course, my own opinion. Grandmothers especially are happy to help out now and again. But they should not be expected to sacrifice their own lives for their daughter or son so that these can make a life for themselves a Pop or Nanna's expense. Certainly Nanna should not stay home to mind the kids whilst both of her own kids are off working full time.
You say, "once in a while," and that is generally fine with many a grandpartent. But taking over the job the real Mum and Dad should be doing is not something they should be called upon to do.