I understand that at 18 or 20 bucks a ball/skein that we are all reluctant to spend the extra 20 for testing, but it is so well worth the end result.
I have found, over many years of Machine and handknitting that wet washing of my swatches, results in SUCCESS. It is better to buy an extra ball of yarn for testing. It can always unravelled and reuse for trim, a 2nd color or one ball project. Or, if lucky and you have a full ball left over from the project, it may be returned to the store for a refund or credit in a timely manner.
In my experience, it has only been the fragile Angora yarns that shed and thinned out.
Acrylics, it didn’t seem to matter unless ironed or hot dryer dried. Air dry, it didn’t seem to make any difference. They too are graded, some are better than others.
Wool, sometimes there were big gauge differences from the washed and unwashed swatch. Wool is graded and spun in many ways, this makes a difference in the end result of any project.
The Superwash wool reacts differently to regular wool. Closer to the acrylics and is dryer friendly. It needs the dryer to revert to original size.
Cottons, too have their quirks as do the other fibers used for textiles.
Mary Anne Cutler,