I thought i'd post this to help anyone who might be either confused about what speed to burn CD/DVDs or who is having issues installing software.
Although CD/DVD writers can write @ over 20x speed, it's not best practise to use high speed when burning large files (IMO 300MB+). It's best to use a slow, if not slowest speed when burning.
CD Audio - I've had no issues burning these at highest speed as you would be burning multiple small files (even after conversion to CDA audio format)
CD Data - Multiple small files, no issues. Larger files (300MB+) at half max speed. Again, this will greatly reduce write (and eventually read) errors.
DVD Data - Multiple small files will have no issues burning at high speed. ISO files on the other hand (e.g. installation ISO for Windows 7) should be burnt @ 2/4x as this greatly reduces the risk of burn errors (undetected) which will eventually lead to read errors during installation.
DVD Video - Due to the large files created when converting home videos or backing up existing DVDs i would recommend 2/4x as this greatly reduces errors (same as above) in video playback (e.g. pixellation, faild playback, read errors, jumping/skipping video.
Hope this helps