I have asked about it and have been told not to do it, as it can cause more damage. My doc said my body will know what to do. Doing the stretching won't prevent any tearing. I have heard others say what your doc said, so I did some research. Based on everything I've read, I'm not going to bother.
My first doc (who was a complete quack) told me to toughen up my nipples and gave me examples of how to do it. Various breastfeeding groups and the What to Expect book says not to do that, because it can cause more damage than good (they do recommend working with your body if you have inverted nipples).
Anyway... my point is, for every person, even doctor, that says something is necessary, there is another that says it isn't--from my experience. I've made it my mission to hear what they say and then research it and decide on my own.
I have heard a good way to avoid tearing is to not push like crazy. Let the baby come down and go back and down again with the contractions. I'm a FTM, so I'm not speaking from experience here