You only have two choices: max speed and 8x. I normally run mine at max speed. That stresses the system the most. If a disc shows good performance at max speed, I can be confident it has good margin for later degradation. Also, I want to and intend to read my discs at max speed during normal use, so I want to run a test that reflects that condition.
Testing at 8x deemphasizes errors caused in part by high-speed reading. Thus, the results of 8x testing tend to reflect more on the quality of the writer and the media alone.
As you can see there is no one right answer.