"Flush" can be done for at least two different reasons.
1. Before harvest as an attempt to remedy over-feeding.
2. Periodically to refresh the reservoir solution.
My opinion is that the first isn't required, but that the second probably is.
It's possible that plants could mostly deplete one or more nutrient elements, and topping off might not supply enough of those nutrients.
Completely replacing the reservoir restores the original nutrient ratios.
Having said that, I no longer change out my reservoir all at once.
Instead, I try to keep nutrient balance by emptying out, and replacing about 10% of the res each day, which roughly equals a complete change every 2 weeks.