Chiropractic, done properly, is never bad for your back. Keeping alignment healthy and all the complex neuro-spinal systems working optimally is always good for your back!
Having someone (or yourself) constantly popping your back is terrible for your back, however. Regularly popping your back can create dangerous hyper-mobility in joints that were previously healthy and does nothing for the ones that are stuck out of alignment and need proper chiropractic adjustments.