I've brewed with both simply repitched yeast and with washed yeast.
Over quite a few batches for comparison, I found no advantage to one procedure over the other so now I just continue what I have been doing for the last 20+years: I save the cake from the ferment, reptich 1/3 to 1/2 of it into a subsequent brew, and either save the rest for another brew (if it's scheduled to happen within a few days)...or else just dump the remaining 2/3 into the boiling wort.
That said, if I were taking my repitched yeast beyond my usual 7 or 10 generations, I would consider washing it. As it stands, I have seen no good reason to go through the trouble or to worry about calculating exactly how much to pitch: the quantities I've stated have never failed me, keeping things worry free, and always producing good beer for me.
After reaching my self imposed re-use limit, I just culture up a new crop and begin the cycle again.
After brewing for so many years and worrying about, trying, and doing complicated procedures at times, experimentation revealed in the end that very often the most simplified procedure produces the very same result with less work and less worry and fussing.
I suggest just experimenting to determine what works best for your situation. And, as b-hoppy pointed out, when in doubt, make a starter.