I just want to emphasize what crashnquilt says about prewashing. I learned this in a class I took on fibers and was surprised to say the least.
Formaldehyde, which, if you're unfamiliar with it, is the stuff they use to preserve dead things. When you see a dead thing floating in a jar with liquid in it, that liquid is formaldehyde. It's one of the most popular chemicals used in fabric because it kills bugs, and their eggs. Fabric is stored in warehouses before being shipped and bugs love warehouses AND fabric.
Sizing is also used in fabric. It's great because it makes the fabric smooth and even looking. When you wash out the sizing, your fabric may suddenly be not so smooth and very uneven.
And just because a fabric is 100% cotton, doesn't mean it has the same weave. Some are tighter than others, some threads are denser, you can tell by feeling the fabric.
And how many people do you think have touchy feeled that fabric before you? Now we're talking dirt and oil.
And dyes do run so washing with a color catcher to catch the running dyes, or using white vinegar, which will keep the running dyes from adhering to the other fabrics and will also "set" the colors in your fabric, is a good idea as well.
If you prewash the fabric you are assured that all the chemicals and whatever else is on it have been removed. After dyes have been released you will have the true color of your fabric. And with the sizing wash out, you will also have the true size of your fabric and you can straighten out any lopsided grain issues you may have as well.