My 3 DC (4, 5 and half, 7) are very lucky and have two sets of GP who live fairly close and love them dearly.
My DPs live 5 mins around the corner and spend lots of time with their GC rather than money on them, only buying for them at birthdays and Christmas etc. My MIL is of the opinion that as a decade older, she has less time left with them and therefore throws money at them at every opportunity.
As an example, the MIL brings presents/chocolate/something every time we see her and the kids don't appreciate it at all, as it is now the norm. I do not want a set of spoilt children, but no matter how many times I ask her not to my MIL continues to buy. She buys things I specifically ask her not to get (ie, clothes/shoes when there is no need or things we already have) and doesn't check with me first (I have to take a Christmas headband set back now as she gave my DD one this weekend, when I have one to give her as a stocking filler).
Past Christmases, I have insisted that all presents come here so that the children don't see a reasonable pile of gifts from one set of GP and then an enormous pile from the other! This was essentially to prevent comparisons being made and affection being bought.
So, my dilemma is do I let the DC open their presents at the houses of the GP who bought them (we will visit both sets on the day), or do I insist the presents all come here to be opened together?