Generally, slower is better, but not TOO slow. When backing up movies, I generally burn at no greater than 8x. That said, some drive/media combos can be touchy the other way...say if you're trying to burn 16x rated media at 2x for example.
If you're using Nero and want to find out how good the disc is, use Nero CD/DVD Speed in the Nero Tools folder, select the Disc Quality tab, and run the scan. The chart you really want to keep your eyes on is the bottom one. Any PI Failures greater than 8 could be a problem for the reader. If it's just a couple that are greater than 8, you'll probably be fine, but if you have a lot of them, or if you have an area of the disc that has PI Failures significantly greater than 8, then you could be in for some trouble playing the disc.
A good rule of thumb is that if you don't have any problems, then you're probably OK, but if you start to experience bad burns that won't playback, ease up on the burn speed.