I guess it really depends on how much you like to iron. I know that when I don't wash, I have to do a light press, but when I wash, I have to iron. And I hate to iron.
If the fabrics are flannels or batiks, washing is a must. Flannels because of shrinkage, and batiks because there may be left over dyes from the batiking process. Rule of thumb used to be that reds / purples had to be washed because of bleeding, but that isn't always the case. You can do a test piece, but I've also been told horror stories about a fabrics bleeding in a quilt even after the fabric was pre-washed.
Also, if you are doing a swap of some type, check and make sure whether fabrics are supposed to be washed or not so everyone's fabrics are the same.