The size and shape of an object can also affect the signal the detector gets. A piece of a pull tab that is curled up may make the detector signal a dime or penny. Same thing with a tiny piece of foil. Sometimes a large deep iron object will make the detector think there's a silver coin. Iron harness/saddle rings will almost always make the detector signal a dime. Something to do with the ring shape. Nails will also have a similar effect. Something to do with their round surface, I believe. Nails usually will make the detector signal iron then silver. Back and forth. You want to dig a consistent good solid signal, but if the object is deeper it may not have consistent signal. Usually if I get three or four swings with a consistent signal and a junk signal mixed in here and there I will dig; especially if the detector shows it is deeper like 4-6 inches. It helps to walk around the object while swinging the coil also if you find some inconsistencies with the signal. If I'm in a park I will put it in jewelry mode and dig decent coin signals including nickels. The parks I hunt I'm only looking for older coins. If I get a good steady signal around a pull tab, sometimes I'll dig hoping for gold. But if the signal is broken around pull tab I will ignore. Now if I'm relic hunting in the woods I will dig everything and anything. This does have it's rewards, but you'll dig lots of junk. Research is the name of the game with relic hunting. You have to be pretty sure some event happened in the the area you're searching. If something has happened and you search long enough you'll hopefully find something related to the event. If I'm around a cellar hole I'll dig everything above iron and then sometimes I get curious and will start digging iron.
-Swartzie