Most of the time I don't need to use a strategy guide, I just enjoy them because A) they're pretty and B) having the map as a reference can be very helpful (I'm looking at you, OOT Water Temple).
Last time I looked at one was yesterday I was doing my replay of Final Fantasy XII, and I couldn't remember what a particular Mark was weak to, so I looked it up.
The only times I absolutely need one are times like in Phantom Hourglass, when you have to close the DS to get the mark on your map. I had absolutely no freakin' clue you could do that (I didn't know that function could be used for a command in the game) so as you could guess, it drove me ****ing nuts. Then in Spirit Tracks, when you have to re-pick up that one rock up off of the lava pillar and move it to the other, I again had no clue you could do that. (I guess I'm too used to games like Golden Sun, where most objects can be moved a total of once, if at all)