I wouldnt call it a myth.A flush is to remove access salt build up from chemical fertilizers, especially when a grower pushes the fertilizers to the limits.In this instance yes you should flush for a couple weeks.If you arent overzelous in your nute feedings then no you really shouldnt have to,but probably isnt a bad idea,chemicals arent good to ingest.If you are using organics you shouldnt have to flush at all
A three week flush wont kill the plant,but at the same time doesnt help it.If your asking about going into week three with a flush then she wasnt ready yet and she should have been kept on the nute schedule till your trichs are almost 25% amber IMO