A few years ago new guidelines have been recommended and I definetly agree with them, especially after visiting a few resin manufacturing facilities throughout the world. Not only that, it is just good practice. Sanitizing water softeners is usually a monthly item in pharmecuetical certified (USP 27) or WFI systems to lessen the total TOC load on the system. Resin has a life expectancy based on the total contact time with oxidizing agents. A typical 8% resin can handle up to .5 ppm for many years. A 50-100 PPM solution for 15 minutes will have a neglible affect on the longevity of the resins life, but will ensure that you are not introducing unknown contamination into your house. To sanitize a new water softener use 3 tablespoons of household bleach per cubic foot of resin to the brine tank and run the softener through a normal regeneration. This should raise the chlorine to approximately 75 ppm. For sanitization after the system has been inoperable for an extended length of time, interrupt the regeneration during the brine cycle when all the water has been removed from the brine tank and let the softener soak for 1 or 2 hours then finish the cycle making sure the chlorine is rinsed out.
Do not introduce a sanitization process to resin that is being used for iron or manganese removal without first acid cleaning the resin otherwise you risk permanently locking the iron onto the resin beads.
To clean the brine tank, a simple soap and water finished with a very mild bleach rinse. Do not use more than a tablespoon of bleach in a gallon of water for final rinsing the tank. The brine tank is where most of the organics are introduced into a water softener and should be cleaned annually. I know someone will say this is overkill, but for the last 10 years I have had to do bacterial counts inside of ultrapure water systems and the softener is always the dirtiest part of the system. A simple regular sanitization keeps them relatively clean and minimizes the amount of contamination introduced into the rest of the system.