I work with my local Project Linus chapter, and we offer receipts to people who make donations of fabric and yarn and other materials. Usually it's someone who's quit quilting or the relatives of a deceased quilter. Roughly half take a receipt, others do not. I don't think any of our blanket volunteers ever ask for a receipt.
Me, personally, I can't even imagine keeping track of what fabric I've bought, what's been given to me, what I've picked up at yard sales, what I've used for charity and what I've used for myself. Like others said, you can't write off your labor, only the materials, and the labor to me is the biggest part. I consider it all a gift from the heart...