Instead of washing yeast I save my White Labs vial and pour some of the starter back into the sanitized vial before pitching the starter to the fermenter. This only gives you one extra vial, but pouring off a bit of each starter gives you virtually unlimited batches. This way you can dump the old yeast cake rather than go through the ordeal of yeast washing.
Though many appear to disagree with me, I will restate that using the whole yeast cake is fine. True, it's over pitching, but my own research indicates that flavor development is only detrimental if the yeast strain has a distinct flavor associated with it (German hef, belgian, English ale). The reproduction phase develops those flavors and with such a high cell count your yeast will mostly bypass reproduction and go straight to attenuation. US-05, however, is a clean American ale yeast and a short/absent reproduction phase won't make a noticeable difference in your finished product. If you choose to wash and store your yeast, keep things sanitary.