CAS basically tells you how many clock cycles it takes for DRAM to perform an action, so the lower the better
I normally look for a max of CL9 for 1600, 9/10 for 1866, 10 for 2133 and prefer performance DRAM i.e. any slower at those freqs I don't touch
1600/7-8 1866/8, 2133/9, 2400/10 2666/11 etc