I have found most "iffy" signals to be junk, but pay close attention to depth and soil condition as that can also affect the signal strength and make good targets sound iffy, I always dig iffy signals that show below 6" and are repeatable in at least one direction.
Some of my best finds were iffy targets and the first time you pass one up and the guy who is following you digs up a really good target (this has ahppened to most of us at least once), you'll never pass another one.
When you are a newbie, dig everything just to find out what those signals really are, but as you get more proficient, you can start to pass on targets that you know (at least to some certainty) are junk.
Hope this helps a little.
Chris