Last few times I tore off shingles, the tarpaper got torn up pretty
badly and for the most part just pulled right off with the shingles
anyway. What was left was so wrinkled, torn, and bunched up that it
would have been a pain to try and flatten it back out. I just pulled
the rest off and started clean with new tarpaper. Lets you inspect the
deck anyway, which you should do while you have the shingles off.

-Kevin