The only game guide I own is for Pokémon Platinum, but I bought that after I had defeated the E4. I wanted it for all of the bonus data it gives you in the back, so that I don't have to open Bulbapedia every time I want to check something.
I'll probably buy the HG/SS guide if it's up to the same standards as the Platinum one, but most game guides nowadays are total rubbish :/