It really depends on the scanning drive and on what you're trying to determine with your scan.
As a general rule I prefer scanning at 32x because it is plenty fast, it is possible even for CD-RW media and Audio CDs, and because it can be done without running at the limit of the drive's capabilities.
Scanning at a low Constant Linear Velocity such as 8x might be more realistic in some respects when scanning Audio CDs, however.
For LiteOn DVD-RW drives I agree completely!
They are useless for scanning CDs.
For Plextor and BenQ DVD-RW drives I disagree completely!
They are excellent for scanning CDs.
NEC DVD-RW drives can also be good for scanning CDs. As always this depends on the particular drive.