I think it varies between hospitals.
I had a home birth with my first, and then ended up in hospital a week later with a womb infection which is when they found I carried GBS. My son was already a week old so past the early onset stage so I was just told to be vigilant for meningitis etc.
With my second, obviously they knew I was a carrier this time and I asked for a home birth again. My midwife was 100% for it and said she could run through basic obs with me and if baby showed any signs in the critical period I would just have to get them straight to hospital. However, you can't control which midwife attends your home birth and none of the ones I encountered were happy to proceed with a GBS+ baby. By chance, I thought my waters had broken at 38+3 so had to go in for a swab to check the next day (they hadn't). The next day I went into labour and the midwife rang the hospital and it turned out they had tested me for GBS (without asking/telling me) when they took the swab and the culture was negative (which I'm not sure about as I think it takes a minimum of 24hrs to cultivate but I wasn't going to argue!) so I got my home birth.
I mentioned it to my midwife this time round and said I wasn't happy with the whole thing as it's transient, so whilst they'll routine test me around 37/38 weeks for it, it could come back positive but I might not go into labour until 40 weeks by which time it could have gone or vice versa so it's the most pointless test in my opinion. Her response was "if you don't look for these things you can't find them
" so that gave me reassurance that she isn't going to pressure me to have the swab, and again, as long as I'm aware that my baby is at possible risk and I'm extra vigilant with the warning signs in the first 48hours there's no reason for me to be in hospital. The fact is, they only check baby every 4hrs? I think in hospital, so statistically, if I notice a change in behaviour or fever immediately (as I will breastfeeding and having a newborn!) I can probably get into hospital just as quick as if they found problems after 4hours if you see what I mean?
I'd definitely ask for a re-swab as late as possible and then you've got a chance of it being negative if that's how they want to play it. It's like anything when it comes to babies, it's your decision at the end of the day and you have to weigh up the pros and cons. I would rather have my baby at home and trust myself to notice if something is wrong and get them to hospital if necessary, than give birth in hospital and distress myself and expose my baby to potential infections with everyone else in the ward, and have us both on forced antibiotics for 24hrs "just in case".