If the yeast cell counts are appropriate for the batch you're planning, you do not need to make a starter. Yeast viability will not decrease much in a week stored in a fridge. I've done this a number of times when my brew day was delayed for one reason or another.
My guess is you'd have to make a starter because you'll be storing it in the refrigerator which to my understanding puts your yeast to "sleep". I'd like to know about this as well because I'd like to start yeast washing
I often don't know that I'll be brewing until that morning, so I don't get to make starters too often. Definitely will help to pull the yeast out when you start brewing and let it get up to temp. I tend to over pitch when I do this too, I guess I figure it will have a better chance of having the correct amount of viable yeast.
Thanks guys im going to over pitch as im making 2 1/2 gallon batches on the stove top playing around with different things corn vs rice in recipes and have already came up with a very good daily drinker that my BMC friends enjoy too . A lemon beer in the fermenter now and a Sam Adams sort of thing is up next with the washed yeast
I've made a starter from a saved yeast cake, here is the link to the math I used which needed two yeast pitching sites. I ended up over pitching though, it's so hard to tell how much viable yeast is really in those cakes without a yeast count