Personally, I'm not a fan of doing a flush unless it's absolutely necessary to address a plant issue. Running such large amounts of water through the pots tends to stress the plants and in my experience, they take a bit to recover. Giving enough water/feed so you get about 10% run off should keep the soil relatively free of salts. A good way to know if you are flushing away the salts is to examine the inside of your pots after the grow is finished. The white deposits on the inside of the pots are the salts. A small build up is normal but if you see a heavy buildup, then increase the amount of run off you get with each feed/watering.