IMO, I do not think you have to do it, but will try and explain why and when its helpfull. Flushing can be done if your plants are having a problem such as nute burn or any other issues caused by inproper feeding, water/soil ph or toxic build-ups in your soil or growing medium. Flushing your plants with water set at the proper PH level will help remove excess nutes or salt buildups. Most people flush plants with three times the amount of water. (3 gallon pot = flushing with 9 gallons of water, let 9 gallons of PH'd water run out the bottom of you pot) You can also flush plants a week or two before harvest to remove any chemical taste that might be cause by the fertilizers you used to feed them. If you are growing organic then flushing isnt that big of a deal. I think if you use RO water that has the proper PH along with organic fertilizers, them flushing will not be needed. I am sure if you search this forum you will find more info than you care to know about flushing or not flushing. I do think its something you should know about if you have an issue that you believe is caused by something you did or didnt put into the soil. Flushing is the only thing you can really do in order to try and get back to ground zero. Tip:Having a soil that drains really well is helpful if flushing is needed. Good luck
Stlmatt