I wash everything that's washable before cutting it.
I've measured fabrics before and after washing - almost all of them shrink SOME. Some shrink a lot. A lot to me is over two inches in width, or an inch in length.
I started measuring to see if it was "worth the effort" to shrink it. It's worth the effort to me.
One of the ones that shrank the most was a Michael Miller black purchased from a LQS. Some of the fabrics that have been the most stable are the VIPs that I purchased at - gasp - WalMart.
I feel that washing before cutting is like wearing seat belts - it seems that many people have no issues when they don't wash the fabric before cutting - but when they do, it's messy.
Most people do get to and from while in a car without an accident - generally speaking, people that are wearing seat belts fare better when there is an accident.
Those that don't are adamant that everything turns out fine - and those that do are adamant that it's a necessary step.