Dunbar is correct (and so is RP), in that you should need to filter only the general water supply to your house. If, despite doing that, you still have iron or other water staining problems with your WM, then the problem has got to be with your water heater tank or WM. You may also have other problems with your water supply that an ordinary cartidge type filter cannot correct, such has hard water…in which case you should seek expert advice. You may need a water softener (ion exchange system).
BTW, may major concern about cartridge water filters is the potential for bacterial overgrowth in those contraptions. If you use them, you should check and regularly replace the cartridges. This can get to be expensive.