Regarding CDRs: I tend to buy 48x-52x CDRs, and burn data at 32x, and audio at 16x.
Regarding DVDs: As a rule, I tend to burn 8x media at 8x, but with 16x media, I burn at 8x and 12x most often, and only use 16x for testing purposes.
It all depends on what your burner and its firmware think of the media you feed it. If your burns aren't going too well, experiment with different speeds.
But if you buy quality media like Verbatim or Taiyo Yuden, you should be good to go.