This is optional for my (HB) MWs, though they discuss it thoroughly. I am actually not declining it, but I may decline IV abx if I am positive, as this is also "okay" with my MWs. My reasoning is this: as a FTM, I have a greater than average chance of transferring, so I would rather have a result on file than none at all. The local hospitals will treat untested women not only as positive, but in some cases, worse (running a whole septic workup on the baby with spinal tap, etc.) And my MWs are fine with a wait-and-see approach if we remain at home. For example, if your water doesn't break until towards the end of labor, you are birthing in the water (which dilutes GBS), etc., they feel like, no big deal to decline abx. If you have PROM and you don't do much laboring for 18 hours or more, then maybe they encourage abx. That sounds reasonable to me, so if I am positive, I will wait and see how labor progresses.
FWIW, they don't feel like garlic is very effective, and they don't care for hibiclens, etc., because they feel that it's just as likely to cause flora issues as IV abx and it's irritating. But just their input.