Industry standards call for a vapor barrier behind or on top of the cbu. Hardiebacker won't be harmed by the moisture, but if there's any that gets back there, it could give you problems with the structure behind (i.e, the studs). At this point, I'd leave it alone, keep the sealer intact by recoating as recommended on the product instructions, and consider wiping the walls down after finishing so there's little chance of lots of moisture saturating and wicking into the backer. I'd run the ventilation fan during and after a shower for a bit to remove excess moisture in the air as well.
Next time, check out www.johnbridge.com for help on tiling. There are a lot of pros (and talented DIY'ers) there willing to answer tile specific questions and set you straight.