I think AS5 is overrated. Any GOOD thermal grease will do. I've used AC5 and the tube that came with my Thermalright cooler(s) (I had a GPU cooler on the last card), and there was NO noticeable change in temps. Plus with AC5 you are supposed to reapply it every year.
Like the Monkey said, unless you are replacing the cooler, don't bother. Or if you are having heat issues, sometimes the stock coolers aren't installed correctly at the factory, very uncommon but it's happened.