The stuff you dilute and the stuff you buy already diluted are essentially the same thing.... If anything, if you buy liquids, you're paying for a lot of water!
The need to periodically "flush" is too dependent upon your overall conditions and feeding and watering regimen to generalize.
For the plants I grow in LECA in non-S/H culture, I water heavily with a dilute (125 ppm N) solution whenever I water (2 or 3 times a week at a minimum), and in my very humid greenhouse, I have no buildup issues.
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