The standard advice always says to wash your fabric before you cut. Full discosure, I don't.
Why? Because I like the sizing. It makes the fabric a little easier to work with and easier to iron. This is kind of controversial because it may cause your finished piece to shrink when its done. I'm sure there are people who have had problems but personally this has not been an issue for me. You can avoid this by not putting the garment in the dryer and the piece will not shrink after its first wash/dryer cycle normally (unless its wool and then never put in the dryer unless you want to shrink or felt it). Poly-cotton blends will tend to shrink more but since I prefer natural fibers anyway, that isn't an issue for me.
This is all just to say, advice is great and can help guide you on your way. But you can work through, experiment and find your own way of what works and doesn't work for you. Your creative outlets should be about having fun! Now go enjoy yourself! That is a rule.