Flushing is used to get rid of a nutrient problem. Its much easier, if you think your plants might not be getting what they need, to simply 'flush' out whats there and start over than it is to try and add some of this and some of that. Hydro growers accomplish the same thing when they change the water in thier resevoirs. If your using good nutreient, good water it shouldn't be necessary. As Thomas said, a week before harvest will help to use up the nutrient stored in the plant so it doen't taste like fertilizer. VV