I am apparently allergic to some of the dyes in fabric. If I don't prewash, my hands get red from handling it (and no, not dye-red ;-), rash and irritation red ). I think dyes are basically toxic, so I prefer to get them out of my house right away. Thus, I always prewash before even putting away stash fabrics.
If you want to use polyester batting and you want some crinkle, you'll still get some even if you pre-wash the fabrics. Just hang-dry them so they don't get dryer shrinkage! Once the quilt is made and washed, they will shrink unevenly around the quilting, making for a bit of crinkly effect. Yes, polyester batted quilts do give you less crinkle.
Otherwise, using washed fabrics with cotton batting gives you the effect, yes. 50/50 cotton poly might be a nice compromise. I agree with you though that certainly for a baby quilt, the lightweightedness of polyester is desirable.