I used GH waterfarm. dwc buckets... I would flush them before every time I would change the rez. It was as easy as pouring fresh water through the medium (at least a gallon per bucket) and draining it immediately. I now use a drip system with hydroton and I flush every week after draining the nutes out. I put 3 gallons of r.o. water through each of the sites. It is truly amazing the amount of excess salts and nutes that are trapped in there. It gives me peace of mind to know that I won't have to deal with bacterial infections, pH fluctuations, or really high ppm. Every time I flush, they DEFINITELY look happier. Like pwizzle said it is personal preference.
As far as the final flush goes, I use AN's final phase about 10 days prior to harvest and one more time 5 days away from harvest to make SURE i get all that shit out of there. Makes for very smooth smoke... Hope this helps bro!!!