As far as I've been able to determine, Work Utility is the be-all and end-all to how effectively you're mining. It's essentially the end result of everything like rejects, luck in finding blocks, etc. I tune to have the highest possible WU, even over hash rate. From what I've read in cgminer's docs, a WU ratio of around 0.92 (in relation to kh/s) is ideal; I'm not sure if this is still accurate though.

Edit: WU is a stat that is slow to build over time. Not something where you change a parameter, watch it for 30 seconds and change again. It can take hours if not days to give a solid estimate.