Well this a subject that can create much discussion but personally I burn media rated upto 8x at it's rated speed and 16x media at 12x & I'm very happy with my results.
Many others here will do the same with maybe dropping 16x media down to 8x for the very best result on important stuff but I don't find that necessary.
With an 8x burner I'd just burn everything at 8x except for 4x media obviously.
The real key is to use quality media in the first place like Verbatim & Taiyo Yuden. Buying cheap isn't worth it in the long run.