I just wanted to address a few things.
According to the CDC (your choice whether you buy their info or not), a mom who is colonized has a 1 in 200 chance of having a child who becomes infected with group B strep. Of those babies infected, 1 in 20 will die from the infection. Others can experience menningitis, organ failure, pneumonia and/or sepsis. If you treat with antibiotics, the CDC says that your baby has a 1 in 4000 chance of contracting group b strep.
Another poster points out the problems with prophylactic usage; true, there are several cons to treating prophylactically, even in the presence of a positive group b strep swab. Waiting to treat until baby shows signs of illness isn't such a great plan either, as there is often little sign that the baby is sick until he is really ill. If a baby becomes ill with (or even if he is suspected of being infected with) group b strep, it isn't just a matter of doing antibiotics, which sounds so benign. Your baby will most likely be admitted to a NICU for 7 to 10 days of IV antibiotics, multiple blood tests, spinal taps, x-rays, etc.
A point that I don't think anyone has made is the increased chance of developing yeast gone wild, aka thrush, when antibiotics are used.
Personally, I would probably refuse the antibiotics, but would get the testing. For the reasons many have already listed. Unless of course I developed a fever during labor (that wasn't due to dehydration or a hot birth tub), or other signs of infection; then I would be okay with getting IV antibiotics. The testing can be inaccurate (if you get tested at 36 weeks, and then go 2-3 weeks overdue, your status of being colonized or not can change), and there are always false positives and false negatives.
I don't think anyone is a bad mom, which ever direction they go. I think it is one of those things that you need to know all sides of, and then make a decision you are willing to take responsibility for. I would be willing to take the responsibility of the small chance of my child becoming ill due to my decision to refuse antibiotics; but that is just me. I respect others who make a different decision.
Hope this helps!