Originally Posted by
Worshaka
In most circumstances it's benefical to gem for the socket bonus using Purified Dreadstones, Runed Cardinal Rubies & Reckless Ametrines. The only time this isn't true is for single, double or tripple blue sockets... in the case of a single blue the difference is usually 0.1 PP, its not until double or tripple that it starts to amount to something significant.
So if you don't want to understand PP or work things out, simply gemming for socket bonuses (assuming its a dps socket bonus) isn't the worst thing to do. Other than that, go to shadowpriest.com and do some reading in the BRGA thread.