You should only flush if you are trying to correct problems.
The trouble with flushing is that you have your rootzone sitting in high ppm's/EC's and you pour vast amounts of water or flushing additive through it completely draining away the ppm's/EC's so you should always follow a flush with a balanced feeding.
But if you dont overfeed you wont need to flush.
Well unless you flush for the last 2 weeks of flowering, personally i dont flush.
J