So up until (very) recently, like as of two weeks ago, I made my living sewing costumes. It's not a cardinal sin not to pre-wash your fabric unless, as
@LuckyMamaInDE said, your fabric has some weird sizing applied that you want to get rid of. Washing might soften up the fibers, which would help the circles of your ears lie smoother when you bag them out (sew inside-out and then turn). Another concern is if you're worried about colors bleeding over time or when they get wet. You should always press before you cut though! You just get a nicer result. "Make it flat n' sexy" is my motto!
How much yardage are we talking? If it's a small piece you can just soak it in some hot water with a little Dawn, swish it around until it's thoroughly soaked, rinse, roll it in a towel to soak up most of the water, and then iron it dry. That's what I do with twill or Prussian tape that needed to be washed before I work with it, but for one reason or another I can't run it through the machine.
Sidenote: I am actually teaching a "Project Runway" - style workshop this week and next to a small class of high schoolers. They are all very enthusiastic but most have zero sewing experience. I'm learning that it's surprisingly hard to slow down and break down stuff like "how to thread a needle" and "how to tie a knot" step by step!
your car will be found safe and sound
