If your drive rips too fast and is unable to read the original, it will slow down and re-read. If you see this a lot, you probably want to drop to a slower speed just to save time. Generally, if the rip is good, the files are good, and the burn will be fine. I have only had one instance in thousands where I had a crc error in the files on the HD and no indication in the rip.
Most discs and drives do best under 12X.