I think it's worth cpu mining if you already have your GPUs going. CPU power is only a small percentage of what GPUs use, you might as well.
The thing about stressing the CPU is that it generates way more heat than when idling, heat that could be extracted from overclocking the GPU a fraction for a greater gain instead of from the CPU at a modest gain.
CPU's under full load typically use between
65 and 150 watts, whereas GPU's under full load typically use between
180 and 410 watts of power.
CPU power usage under a heavy load is a large percentage of the power used, unless you are using a dedicated mining rig with a low power (low performance) 35-45 watt CPU.
I personally don't think that it is worth it. Maybe in the short term it is worth it, if at the next difficulty or three it jumps so much that mining is uneconomical get as much as you can while you can by any means possible.
