There's no real advantage to burning at a slower speed. I wouldn't unless doing so results in problems. Try buring a couple rewriteable discs and see what if anything shakes out.

You can find pleny of discussion on this topic over at DVDHelp. The topic pops up time after time. I followed a few threads when still burning CD's years ago. Two camps. One use the arguement if you burn at too high a speed the audio may be of less quality because some bits lost need to be compensated for by the DVD player which within limits it is perfectly capable of doing and unless you got ears like Mr. Spock, you'll probably never hear the difference is basically the position of the other camp.

On the video side any problems from trying to burn too fast will be obvious. Again, the firmware of the particular DVD burner you're using shouldn't take the video stream faster than it can be written to the disc so its mostly a none issue.