This is highly questionable these days and the source of much discussion.
With a modern , quality 16x burner , quality 16x media then most find that 12x burning gives great results. With the latest 18x & 20x burners excellent burns are being achieved on 16x media at 18x or 20x.
However, it's entirely down to the individual as to what they want to do. Me, I burn my 8x media at 8x and 16x media at 12x. I only use Verbatim media or media that has a Taiyo Yuden dye like Plextor 16x +R these days .