I can be found to make several test blocks if I don't get things perfect the first time around. I guess it also depends on how badly I want to make the block. I am in a swap that I just turned down the block I had planed to do because after 3 test blocks I could not get it perfect. So it was onto another block design. The test blocks will be something for another day.
I only pre-wash occasionally, when required for a swap, or for deep reds, blues, or purples.