Actually testing at anything above 0x will test also the drive, and not just the disc.
The dvd disc is a rotational medium.
It needs to be spun when read (excluding off-line electromicroscopy scanners for now).
However, the more you spin it, the more you get RF jitter.
RF jitter is what causes PIE/PIF/POE/POF errors when the signal is being read back.
So, the amount of errors will vary as a function of read (rotation) speed (and drive): more speed almost always equals more read errors.
So, what is the optimal speed to test at?
If you want to stress the quality of the disc more than the reader, then the lowest speed is theoretically the best.
However, as pointed out, that doesn't take everyday use into consideration.
In everyday use, discs are being read back from anywhere between 4x to 16x (depending on drive, settings, firmware, disc and which part of the disc is being read).
So, ideally, to test for worst case REAL LIFE compliance, discs should be tested at 16x.
However, that doesn't take into account the fact that the higher the read speed, the more drive/reader dependent the score is.
So, while one drive might be able to read the disc at 16x ok, another might fail utterly so.
Is that disc then good or bad? How is the burn quality?
One scan with one drive cannot prove anything about the general burn quality. Just compatibility with that one drive.
Also, again scanning at slower speeds doesn't stress eccentricity issues so much as as scanning at higher speeds. The rising mountain type of read error graphs is most of the time related to disc eccentricity issues and the drives inability to cope with it.
So, to sum it up:
There is no one right speed to do quality scans at (theoretically and practically), but for practical purposes it is useful to standardise on ONE speed, rather than getting wildly varying speeds from everyone (making the rough comparisons between scans even more impossible).
At CD Freaks the recommendation has been 4x (or sometimes higher for some other models, if 4x isn't doable).
However, this does not mean that scans at other speeds are worthless. It's just a way of standardising the scans at one speed, rather than getting different speeds scans from different people.
PS My personal opinion is that a good disc (i.e. a mechanically properly constructed/non-eccentric/evenly thick disc, with nice optical characteristics, no birefringence, no dye or sputtering unevennes and good burn definition) will scan with low error rates in ALL drives regardless of the scan speed. However, consumer dvd burning isn't that good yet. Therefor it's usually more practical to talk about average everyday disc compatibility (i.e. reads back at 4x in various drives) than ultimate burn/disc quality (reads back at 16x in EVERY drive).