Nope- Not ever!

MIL & Her partner live overseas, and my kids don't spend time alone with FIL and his wife so we dont have to worry about about them- but if they ever were to spend time with them I would make my ideas on smacking extremley clear.
As I child I was very very rarely smacked, and I knew when I was that I had pushed the boundaries wayyyyyyyyy to far- Mum and dad have the kids alot, and mum has already said that she would never smack the kids as she knows that is not the way we discipline.
In all honesty I dont think that it is anyone but a parents job to discipline a child when they are present, and when your child is in the care of someone else, I think that person should follow the discipline methods that you choose as your family.