I have yet to find "official" guidelines. This is just me, (and I completely respect and understand someone who would abstain from drinking while BF) but I've been having a small glass of wine a couple of times a week, with dinner, since DD was born, and haven't given much thought about it. I don't feel the effect of that amount of alcool, especially not with food, so I'm not worried about it. I have friends who don't drink at all while BF, and other who do. None of them are getting sloshed, though, we're taking one glass with a meal.
My Dr. is aware of this (during my regular check-up, she always asks about eating and drinking habits, and she's aware that I'm EBF) and didn't say anything one way or the other.
But again, like I said, I'm not aware of what the official guidelines are, or even if there are any.
ETA: I remember from my DDC having a discussion about drinking during pregnancy, and it did come out that this was a cultural issue; in Europe, women have no issues having ONE drink every once in a while, while here, in North America, the bottom line is complete abtinance. I find this a very interesting thing and my guess you'd find similar differences about drinking while BF.
I guess I should add that my family is French, and while I would (personally) never drink during pregnancy, when I came home from the hospital with DD, my mom was waiting with dinner and a glass of wine for me!