Depends on your reader. High quality anti-jitter models like Yamah can read at maximum speed with 100% success for unmarked original media.

The problem is for scratched media. Slower read speeds improve the chances of avoiding a read failure - but you'll only be lucky if the media is superficially scratched.

If you set CDRWin to stop on read errors and if you are burning via the hard drive, you won't waste any CD-Rs if it reads without error. Burn at no more than 4X is best advice.