It is always said not to drink when taking antibiotics, but this is more due to the fact that they won't work as well. You are slowly killing off an infection (if one exists) on antibiotics, and if you drink excessively on them it would be like having to start the course all over again. If it is for prevention then you are lowering your immune system by excessively drinking, so could make things worse in maybe getting an infection. Also some people do get a little sick combining the two.
As for drinking after a graft i would say that you should get it checked out just to make sure. I would go with the reasoning though as with most surgery, that it can as Jay said cause a decrease in blood pressure, but also that you could do some very silly things when drunk! Also in all seriousness your body does need time to recover from surgery and be healthy for it to heal, so any alcohol could slow this process. I would check with your surgeon, but am sure that he may well say that in a little while some amounts of alcohol would be ok, just don't drink too much!!!
I left it for three months before i drank, but i am very sensible and haven't overdone it just in case!
Sweet X x X