I didnt wash my finished pieces until I worked a few afternoons with a fabric conservator.
She reminded me that the pieces we see in museums now were all washed in their lives. This removed the natural oils from our fingers that seep into the fabric as we are working. This wont be seen on a piece for maybe the first 50 years, but if parts of the fabric absorbed more oil than another it will begin to age differently, and this might stain.
She also reminded me that modern fabrics are coated in a preservative, that helps prevent thread breakage, and discolouration and mold during storage. Museum age stitching was never stitched on this kind of treatment. We dont know how fabrics will age with this stuff left on them. Only time will tell.
I always wash cotton based fabrics now before I start stitching. Then if theres some shrinkage it wont distort my piece. I use DMC, Anchor, or Madeira mainly. But to be honest Ive used all kinds of thread, Chinese silk, rayon, hand dyed stuff, and never had too much of a problem. I have started to use those color catcher sheets you get in the supermarket. They work great in the washing machine, and I figured the process is the same in the hand bowl.
Once Ive washed a finished piece, I roll it up in a clean towel. I dont wring it. Then I either iron the piece dry though a tea towel or two, or I stretch the piece on a pinning board and let it dry.
If I live another 50 years, Ill let you know if theres a difference between my first pieces that I didnt wash, and my later pieces that I did!
The OP says that "every completed project should be laundered" and that statement is MISLEADING. Many hand-dyed fabrics will run and/or discolor with washing. I work with a lot of silks and over-dyed floss which will also bleed when washed. Pre-rinsing is not a good option, because the colors of the floss may fade out from the rinsing. If Im stitching solely with DMC, I feel confident with washing my piece if the fabric has not been hand-dyed. But I always wash my hands before stitching, and every 1/2 hour or so, to try to keep the piece as clean as possible.
chekccard