I think there are 2 different issues. One is when you're around (e.g., your parents came to visit you in your house) and the other is when s/he is at their house (without you) or they have come over to babysit.
In the first case, you should be in control - you decide when it's OK to pick her up, feed her, change her, what to dress her in, etc. The message to your parents is, "It's important to me to be able to raise my baby my way." Even if there are 'mistakes' made along the way, they're yours to make... You will need to be consistent in giving the message, "Thank you for your suggestion, I'm the mom and I'll make the decision."
In the second case, you've handed over control to them. Some of the decisions are still yours (e.g., what she's allowed to eat, when she should be in her bed, etc.) but some things are out of your control. It's OK for there to be different rules at your parents house than the ones at your house. If you won't allow jumping on the couch and they do, that's OK. If they don't allow climbing on the table and you do, that's OK too...
I hope that helps... my parents almost never interfere & my husband's parents have to say something about practically everything (e.g., if she's wearing socks, "Aren't her feet going to be too warm?" and if she isn't, "Don't you think she needs socks?") So we have just repeatedly said that we can handle it. You'd think that with our 5th child they'd get the point that we kind of know what we're doing...
Rachel (mom to 5 kids ages 21 months to 14 & expecting #6)