Make it just to be sure the yeast is still alive n kicking n healthy. AND you get more yeast !My washed yeast was washed on 9/15/ 11, soo I know now its ok!
Hope this helps. Many others will give a better and more detailed reason!
I've done both. If it is fresh I will use the slurry. If it is older I will make a starter. I also have frozen some yeast which will require a step up starter. I have not used any of them yet.