It is probably not necessary to sanitize the water heater. The high temperatures should keep the microorganisms at bay.
That being said, I do sanitize mine.....people say that I am anal.
1 think that you need to consider is the water heater is a holding tank.....there is no way to flow all of the chlorine/water molecules in and out of the tank. Start with an empty tank to make sure it gets filled with the chlorine solution, and drain the tank before the fresh water flush.....again, there is no way to flush all of the chlorine solution out of the tank without draining before the flush.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263