Usually you get the best quality burning media at 1/3 to 1/2 of the maximum speed of the burner. If this is the maximum speed that media is good for, the next speed down is usually best. You can test to be sure.
Example: my main CD-RW drive is 52-24-48. My DVD burner burns CDs at 24-10-40.
If I have 48x or 52x media, I should probably burn at 24 or 32x on my main drive, but since 24 is the maximum speed on my DVD burner, I will probably get best quality at 16x.
If I have 16x media (actually I do, some nice Memorex black CDs that I seem to be using slowly ), I wouldn’t burn at 24 or 32x, even though my burner will let me. I will burn at 8 or 12x for quality.
With DVD, though, I find it isn’t so clear. My DVD burner is 8x + and 4x -. I find that it burns 4x media beautifully at 4x. It isn’t necessary to step down to 2.4x, although I might find that it is slightly better. In truth, I have only burned one 8x DVD and it sucked (although it was 4x media, so perhaps the drive was overly optimistic about its ability to burn the media at that speed).
If you need to burn fast, feel free, but I think you’re best to avoid the very fastest speed of a CD burner unless you really need to burn quickly.