what do you do with them? do you actually keep them? have you ever needed them? i typically find them balled up in the bottom of my purse, and i throw them out about a week after i've mailed the books. just wondering if i should  be a little more careful with them?

I'd hold on to them until the book has been marked received and then toss them

Before I used PBS DC and / or postage, I would hang onto them until the book had been marked received, just in case I needed to use that number on to see how far the book had gotten.  However, I don't need it now that I use the PBS services.  The DC number is in the transaction archive for the book under "See details."   I just recycle them.

 I tell my PO guy I don't need them, he always offers receipt  even when I don't use DC.  I tell him to save his paper :). Now that I use mostly PBS DC, I don't need them anyway.

I keep them until the item has been marked received--be it here, eBay, posh, whatever.  Until the other side has acknowledged it, or a few weeks since delivery, I leave them in a small pile under my monitor.  I usually don't have more than a couple at a time.