GUIMiner has the option for you to be able to choose how many cores of CPU you want to use while mining. I was wondering (I have 8 cores to choose from for my laptop), assuming my laptop would be running the GPU anyway, is it worth it to have the CPU running, or is that just a waste of money in electricity?
CPU mining is far too inefficient to justify using it. The only CPU mining I do is with Bitcoin Plus and that is only because I utilize computers that are not mine and therefore, at the least, there is no cost to me (whether or not there is a profit is an entirely different story).
If you are using OpenCL, I do not believe that checking the affinity boxes will have much, if any, effect on the GPU performance.
but it is assumed that the gpu is much faster for mining, not?
Yes, it is infinitely more efficient than a CPU at mining. It is very good at doing grunt work and repetitive tasks.