I piece using Prescencia 60/3. On the cone it seems to be pricey, but if you do the math, it is comparable with other threads when comparing $/inch. I can't get it locally so have mail ordered it. For piecing, you want neutral colors, you don't need to match your fabric colors, which is what I thought initially and consequently ended up with lots of thread in colors that will take a while before I need them. The 60/3 is a fine thread but also very strong. And makes it easy to sew a good scant quarter inch seam.
I do prewash fabric. And for this reason, have not bought many precuts as I'm certain that I don't want to prewash and then press those. When I do use the ones I have, I probably won't prewash the precuts.
Cheers, K