Ever notice the manufacturer doesn't always get the fabric folded and rolled onto the bolt square? This is one of the main reasons I choose to pre-wash. Pre-washing gives me a chance to re-square it before I refold it. It can be an inch or more off in a single cut.
Here's what I do:
Fresh out of the dryer - hold it selvage to selvage let the fold fall. Is there a diagonal "bubble" in the fabric? If so, slide one of the selvages right or left - when it hangs completely flat, it's right.
Now look at the cut edges again. 85% of the time, I've lost some usable fabric from the original cut. Can't discover this unless you pre-wash.
So strips cut from an unwashed piece, may not actually be on the straight of grain and sometimes this shows.
Hope this helps someone...