S.M.
Agree, agree, agree... then do it your own way. Example -
MIL: "You beat him when burping."
YOU: "Haha, he'll love to hear that story when his is older."
MIL: burping with pacifier
YOU:: walk over, kiss baby, and take pacifier "Uh-oh, Grandma forgot to take this out."
MIL: no burp
YOU: Take him after after she is done. "Thanks Mom" and beat his back :)
or YOU: "He's not cooperating today,is he? Would you like me to take him?"
You shouldn't have to compromise your approach to parenting, but not everything needs to be a battle either. Of course, draw your line at what you feel is dangerous. A friend's MIL used to leave the baby laying on the middle of a bed during naptime. NOT OK! But it doesn't sound like there is anything dangerous going on or that will impact your son's overall experience. Pick your battles, but be kind and take the approach of "Oh, I think you forgot, we do it this way...." Or "The pediatrician asked us to..." Let her love her grandson, and realize she is probably very excited about him and wants to talk about the kind of mom she was to your husband. She is not necessarily being critical, but trying to be involved and helpful. Engage your husband to back you up.