If they were nice I would tell them to keep the change, but I don't think its considered a thing your supposed to do. I would do it since there don't seem to be many people selling newspapers anymore.
If you buy a newspaper from the person standing on the street corner ... Do you give them more than the paper costs ? (tip)?
I agree with Cool Spot and Happy 2 Be Here 2. I would. I've been a tipped empolyee/empoyer for a good part of my life. It is appreciated if the guy or gal is considerate and/or nice. It's a nice gesture since they are trying to make a living too and giving you the conveinence of not having to walk to stand or to the store to buy one.