With modern CD writers the highest speed generally does not produce a higher BLER (BLock Error Rate), basically writing audio faster does not increase the errors hence no cracks.
If this does occur it is generally down to poor quality media.
Ive writen audio from 2x to 32x (using branded media) and never had a problem with playback, also the error level on the disc was not drasatically different
So my advice is burn at the fastest speed you can
