I do not hand piece but I do hand quilt. Opinions on prewashing are as varied as the people on this board. I am in the prewash camp for all fabric that comes in my door (disclaimer: I do not buy precuts which should not be prewashed, such as jelly rolls, honey buns, charm packs, etc)
I prewash to:
Remove commercial sizings and chemicals applied to fabs.
Preshrink
wash out any excess dye or to set unset dyes using Retayne in an effort to eliminate any issues with bleed.
I feel prewashing is just a good pre-emptive step no matter what you are doing with the fabric