Scanning CDs at max is deffinitely the toughest test to pass, errors are generally higher the faster you scan. Some drives will have minor glitches at max speed, PUH slippage, and other little quirks that make the scans come out a bit odd. I generally scan at 40x, since the results are very close to max, and my drive seems to behave a bit better.
It's really personal choice, but remember that lower speed means a better scan so take the results with a grain of salt. Whatever you do, be sure to do it consistantly, and don't compare scans done at different speeds. Also don't compare scans done in different drives, expecially DVD burners.