WH = wall hanging? My best guess.
I too prewash all fabrics and usually wash the finished project. I was the very upset recipient of a wall hanging for my infant son some 36 years ago and with his asthma and allergies was told to launder everything in his room at least weekly --- suffice it to say the wall hanging shrank and bled and ran all over the place. So, to avoid any9one having the same problem I prewash. And I use warm water to do it in. Cold water will often cause embroidery floss dyes to run where warm won't.