i'm going to slightly disagree with everyone here.
k/bb is a great statistic, but so is whip. if you're already using whip, then just k's makes more sense.
here's my logic: it's good to have a mix of cumulative, average, and sometimes negative statistics to balance everything out and make no particular strategy great other than simply taking great players.
if you have k/bb and whip, you're using two statistics that reward relievers that will have high k's, low walks, and low hits. if you use only k's, you're encouraging pitcher churning and plugging in long starters who may be wild, but manage to get some k's along the way.
if you use k's and whip together, you're rewarding top-of-the-line starters, moderately rewarding quality closers, and hurting/helping mediocre starters, which in my mind is how baseball (real and fantasy) should be set up.
so... long story short, the key is not only in selecting good statistics, but in selecting statistics that work together to encourage quality players instead of just a particular fantasy baseball strategy.