it`s a good idea to change nute solution every week or 2, and flushing is not only for, just before harvest, if you put too much ferts in the solution, it`s a good idea to flush it out with plain water.
but yes, flushing of the plant is usually during the last few weeks of flowering stage.
and remember, when adding water to resevoir between changes, only add ph balanced water, not nutes mixed in.
hope this helps