CPU mining is obviously much slower than GPU, but since my machine is always running anyway, a few extra hashes couldn't hurt.
Just a tip I thought I'd share: if you add 'start /idle /b' to the beginning of your batch file, the process will start at a lower priority, and will only use the resources that are entirely unused by other apps, as opposed to starting with the same priority as other apps you're running. (The '/b' means run in current window and don't open a new one)
Also, you can specify how many threads minerd.exe uses, and specify one less thread than you have CPU cores, making sure there's always one core available to handle the GUI. Do this by adding '-t 6' at the end (if you have 4 cores with hyperthreading).
Here's my batch file. You're welcome to use this example and mine for me as long as you'd like (you know... just to get the hang of it).
start /idle /b .\cgminer-3.8.4-windows\CPUMiner\minerd.exe -o stratum+tcp://spare.ozco.in:9333/ -u b00m3rang.pctcpu -p asdf -t 6
I also use the 'start /idle /b' for cgminer and cudaminer, since it does use some CPU power to interface with your ASIC/GPU. It doesn't seem to slow down the hash rate at all, and I can still play Guild Wars 2 on low settings while mining 3 different ways.
