Some of us are adamant about having everything washed/shrunk before cutting it.
Others are equally adamant that it is not necessary to bother with doing it.
My personal experience has been that it's worth the effort. It's kind of in the same category as using seat belts - most of the time the fabrics are okay enough to use "as is" - but every once in a while I come across a stinker.
Same for seat belts - most of the time I get to and from wherever with no problems. Once in a while one doesn't.