For that I would say you really gotta base it on the personal situation. Depends alot on What your hunting for, where your hunting and what your finding.
Like if your looking for relics in old places, you should probably dig most of the iron signals, but in somewhat newer areas iron signals will probably turn out to be junk.
Now if your at a decent spot thats giving up some nice stuff, its probably best just to dig some annoying iron signals just to get em outta the way. You may find an iron item could be masking something good close to it.
I guess just a little trial and error and see what your pulling
It really depends on where i'm hunting. In a wide open field the detector will be in all metal but just about everywhere else the will be at least a little discrimination.