This is mostly true. It's recommended that you don't go into, say, The Pitt, before finishing a quest like Reilly's Rangers, as that one can cause a problem if you don't get to them in time on the rooftop (they become hostile toward you). Finishing it by going to Ranger Compound can wait as long as you want, though. This is just one of maybe three examples (and the only one I've had happen to me thus far).