It does seem to matter what speed for me. With 74 min or less, I can burn at 32x, but with data that is more than 74 min (related to the media itself most likely?) I cannot burn faster than 4x without creating coasters out of 1 in 3 discs. Once I dropped to 4x, I have 100% success rate for burning. The quality of the resulting discs doesn't change, but the rate of failure was noticable based on speed alone. I say try it at max and see if it works - if it does, you're all set. If not, drop it down until you find the rate that works best, sticking with the highest rate that your system will do.
With data, it doesn't seem to matter what speed I write at, but then I haven't done any discs with enough of data to fill a disc completely yet.