As Dee said!
There are exceptions however; All the 8x and 16x rated media I have burn even better at 6x on my Plextor PX-712A. Maybe this is because 6x is Constant Linear Velocity?!
Anyway, on my NEC 3500 (not a 3520) my experience is that 8x media burn best at 6x or 8x (sometimes 12x) but worse at 4x. At 2x I even got a coaster with Verbatim media!
My NEC 3500 burns 16x Verbatim +R media better when reducing the speed to 12x however.
For CD-R media it varies, but 52x rated Verbatim DL+ media burn equally well at 32x, 40x and 48x (max) on my NEC 3500. The same media on my Plextor burn best at 16x (Constant Linear Velocity!).
So in general using the media's rated speed is best, except you might get a better result by not burning at the drive's maximum speed (too much vibration).