I was just reading an article about a university study that ran several chromatography tests on plants that had varying degrees of flushing done and they all came out chemically identical which puts the nail in the flushing coffin. We all know that there's a lot of old wives' tales around about growing cannabis and like plenty of other things in our lives a personal experience of correlation can lead us to believe firmly in causation. That's where superstitions come from.
The article also went on to show that the flushed and unflushed plants also had roughly equal weight and THC content, which means that while you're not getting anything extra out of flushing them, what you ARE doing is not wasting the nutrients you paid good money for on a plant that won't give you anything extra in return for them.