I use paper but not after the whelping.
After the whelping I use blankets with newpaper UNDER the blankets. Sometimes if I can find a deal on it, I cut indoor outdoor carpet to fit the box, and put blankets on top of that that I can wash every day. Then I change the carpet every 3 to 4 days, or when it gets soiled.
I don't like paper because it does not offer puppies enough traction to move about effectively on, and because of the newsprint on it, and because of it's sogginess when wet.
I do sometimes use it in expens, etc, when puppies are older, but not often in the box.