AS5 is not very conductive, but it can
and often does
cause problems if you are sloppy with it. I've shorted GPUs before by getting AS5 on the substrate mounted capacitors/resistors. It's very annoying to find you've done this after the card is assembled, but I've never encountered permanent damage (cleaning the stuff off well and being more careful the second time fixes it).
Anyway, it's perfectly possible to use AS5 on almost any component, as long as you are very careful to not get it were you don't want it. Consider masking the area around the GPU with tape while applying it, if you don't have steady hands.
My prefered method of applying AS5, or almost any other TIM to bare dies is to lay down a very thin layer, as mentioned. However, I take another step to ensure that no air pockets form by placing a very small dab in the center of this thin layer that will spread when the heatsink is mounted.
Just the layer might allow air pockets, unless you are very careful mounting the HS and have very smooth surfaces, while just a dab won't spread to the corners of a square die unless you use way too much. My method is a compromise.
Originally Posted by jetOne
But would AS5 be ok on my CPU?
Yes, unless you've already got something better.