I think that an "iffy" signal should not be used to determine whether or not a target should be dug. To me, the term "iffy" descibes the chances of an item being junk or good.
Iron signals are not iffy to me. If I get an iron signal, I dig it only if it is a really strong, solid, repeatable signal. I'm in this for coins (hopefully old) and jewelry, not relics, screws and nails.
The signals I consider to be iffy are the ones that bounce from foil to nickel to pulltab, and from dime to penny. Every time I get a foil-nickel-pulltab signal, I hope for a small gold ring or earring, but brace myself for a wad of foil, pulltab, zipper, or fishing tackle. Same with the dime-penny signals, which can be a dime, penny, or a coin tilted on edge, but mostly turn out to be washers, snap buttons, or other copper junk. I dig these signals all the time, and every once in a while, something good pops out.
Don't let an "iffy" target ID readout make the decision to dig for you, if you do, you will be passing up the 1 or 2 good finds out of 100 on these iffy signals.
HH!