Seems to be two schools of thought about this. Flushing is to remove salt buildup to avoid lockout in early flower which is common if you've been overfeeding and/or not feeding for 20% runoff. Some feel it's easier to flush once instead of measuring runoff ppms to detect salt build up and adjusting nute strength/runoff to avoid lockout. Others feel flushing is stressful and they spend more time dialing in nute strength and runoff volume to avoid salt buildup.
You have to figure out your style. If you're new it's probably better to flush. It takes some practice to monitor runoff ppm and correlate it to soil ph acidity and nute lockout. But, eventually you may want to try not flushing and learn where lockout occurs so you can dial-in your strength and runoff volume.
I don't flush. I monitored my runoff ppms to find where salt build up reaches lockout. For me, 2500ppm is where the soil ph is about 5.2-5.4. This is where lockout starts for me. Now I know how much to feed (strength and volume) to stay below that level. I spot check the runoff ppm in flower. Sometimes I'll increase runoff if the ppms are 2200.
