I've never had a problem with stale soda smells, even root beer, after rebuilding a keg and replacing all the O-rings. That includes the lid, dip tube, and post O-rings. The trick is to disassemble everything, clean and sanitize each part individually, then reassemble and sanitize the inside of the keg.
Sometimes there's still a hint of soda left with a newly-purchased keg, but it's never showed up in the beer and doesn't last after the first keg of beer.