NPCs in Premade Settings

In the comments of my previous post on "advancing the timeline" in Greyhawk, commenter scottz made a very interesting point:

"Characters/NPC's are canon 'mountpoints' as much as maps or gazetteers are, IMHO."

I take this to mean that, if one is running a campaign in a premade setting such as Greyhawk or the Forgotten Realms, one expects to interact with, or at least hear about third-hand, some of the notable non-player-characters that have become associated with that setting over the years.

I agree wholeheartedly with scottz on this point; notable NPCs from the setting are as much a part of its ambiance as its notable dungeons and the names of its kingdoms.

In Greyhawk, this would mean the likes of Melf, Tenser, Mordenkainen, Sir Robilar, and so forth. In the Realms, the quintessential NPC is of course Elminster, but being taken on as an agent of the Harpers to thwart the machinations of the Cult of the Dragon would certainly qualify as well.

Now that I'm going to be restarting my own Greyhawk campaign, my thoughts naturally turn to how I can make it an exciting and enjoyable experience, and the role of these sorts of notable NPCs naturally springs to mind.

I've actually done this in the past with great success with some of the minor characters encountered in the Gord the Rogue books (and who are known from GH lore as well) such as Biff the halfling, who is a henchman of Melf, and it was a complete blast.

But I wonder how many DMs out there do this as well? Outside of published modules (because, naturally, anyone who runs the Temple of Elemental Evil is going to by definition have their PCs encounter notables such as Burne and Rufus), how many people who run Greyhawk or other premade settings also make a point of using some of the notable non-player-characters when they do so?