Humor is a matter of style, and style should be fairly consistent from doc to doc. But I actually love humor in technical writing, because I think that humor (done well) is a great way to make something memorable. Make someone laugh unexpectedly, and they'll remember it. Humor is engaging—and when you're writing about dry topics, finding ways to keep the audience engaged is half the battle.
You certainly can go overboard with humor, though. And unless you have a natural flair for jokes, it can be genuinely hard to write funny material (force the joke, and you kill the joke). As always with writing, it's a balancing act. [http://www.mackie.com/support/downloads/manuals.html|Mackie], by the way, is really good at balancing information, instruction, and humor in their product manuals. I'd look to them for a good reference.