GBS status can change even within the pregnancy. You could be positive at 30 and negative at 34. If you are worried about having AB's going into your newborn, you could consider some natural methods such as garlic cloves inserted into your vagina.
There is a risk, but it is important to keep risk in perspective. Of mums who don't have any ABs in labour only about .5% of babies suffer from the infection, and even fewer are fatal. You are well within your rights to refuse ABs in labour.
It's really important to speak with your care provider about any testing they do, before agreeing to the test, as you can see, the results of these tests can have some quite serious repercussions for yourself and your baby.