ABSOLUTELY! Prewash everything. Last time I did a test for a local quilt shop, Moda fabrics shrank from 44" to 41" and the Benartex flannels (which came with a label assuring us that there was no need to prewash it) shrank to 39".
I am more concerned about the chemicals - those finishing agents, especially the ones used to give fabric that heavy slithery feel, and the pesticides used at the border when fabrics are imported. UGH. I'm not particularly allergic, but my eyes sure itch badly after I have been in a poorly-ventilated fabric store for any length of time. And because our hands are on that stuff for so long, I think it's very important to wash away the chemicals.
If you don't prewash your fabrics, PLEASE PLEASE PLEASE be sure to let the recipient know that you have not done so, especially if it is a child's or elderly person's quilt.