I would not worry about the metal in the valves we might put in. When you look at most of the water supply in this country, it is metal. From the pump in the ground to the water storage tank, through the pipes and on into your house, it is likely to be metal. You most likely have a metal faucet wherever you get your water for the tank. The water may take a week or more of sitting in metal before it reaches your house so worry about it passing a metal valve is not really worthwhile.
+1 on the above unless the valve will be siting in the tank it's effects on the water are entirely negligible.