Well, the anitbiotic-bcp thing hasn't ever been proven by studies, actually...it's just anecdotal evidence of women who got pregnant while taking antibiotics and the pill at the same time and a theory that due to liver metabolization the pill wont work as well when a woman is on antibiotics. Another theory I've heard is that this is true in SOME women. That for a small percentage of women, their body might metabolize the pill differently while taking antibiotics.
I've been told by my doctor that only TB antibiotics effect the pill, so that back-up isn't needed. However, since there's still that theoretical risk of efficiency being decreased, she advised me that if I wasn't comfortable with a possible (though unproven) risk, to use back up for the duration I was on the antibiotics and for seven days following.
There are actually studies out there that prove that a number of antibiotics do NOT effect the pill, such as Cipro and Macrobid.
Also, the theoretical risk is small. Like, the efficiency would go down from 99.7% when taken perfectly to say, 95%. Which is still very effective, just not AS effective.
All the same, I don't claim to know everything, so I guess the short answer to my long-windedness is to use back up during the time you're on the antibiotics and seven days following if you're not comfortable with a theoretical risk.