My suggestion is forget washing yeast. Like you said, you never really know how much you have, plus it's all gunked up from the trub.
The better way (in my opinion) is propagate the yeast (from a fresh yeast pack) by making a large starter, splitting it, save part and pitch some. Continue starting and splitting every time you brew. The process is practically the same, except without the washing, and you know exactly what you have.
Also, if you're an AG brewer, save some of the wort for your next starter, or use the end runnings and that way you're not spending money on DME for the starter.