Most of what I know about Greyhawk I learned from the City of Greyhawk boxed set (which I sadly gave away years ago.Currently the only Greyhawk materials I own are the 4 part Paizo produced poster maps, Greyhawk Adventures, and the 3.0 Gazetteer. Okay technically I own the atrocious attempt at humor that was was the WG7 Castle Greyhawk. We don't like to speak of that module though.
While I played many early games in a homebrew setting, my friends and I have spent most of our time adventuring in Faerun. The Forgotten Realms books were always more readily available, so it became our default setting. This of course can lead to potential conflict, if a player brings up "in this book so and so says." However, we typically played with the caveat "this is my take on the Realms." I don't have to worry about any of this if I use Greyhawk. I have broad strokes of the setting, which means I can flesh it out however I'd like.
As I mentioned yesterday I'm thinking about putting Death Frost Doom in my next game. While I'm not going to force the characters to push the doomsday button in the adventure, I have big plans if it happens. The followers of Iuz will seek to gain control of the undead to replenish their armies. Yes having an army of the dead isn't the same as an army of fiends, but it can still be useful.
Oh and either way I definitely want to use the Warduke. Whether he gets an army of undead or not, Iuz and the Horned Society (and the Warduke) will have some part in the campaign.