First, you asked "usually" and that calls for some clarification.
To me, there are two basic groups of metal detector operators, and it is especially focused upon the type of ground they hunt and their loyalty to very accurate target ID or VDI readings. I call these two groups the following:
Traditional Coin Hunters and Avid Detectorists
I separate the two classes rather simply in this manner: A Traditional Coin Hunter is frequently associated with an 'average' or 'typical' hobbyist who mainly searches for coins; hunts 'common' sites, like parks and schools and yards; and 'generally' relies on very accurate visual (TID & VDI) and audio (Tone ID) responses to help classify the most typical and desired target they want to recover. Yes, even I take the Traditional Coin Hunter approach at times in similar sites for leisurely hunting.
An Avid Detectorist is a person who takes a more direct and total approach to target recovery, much like a dedicated Relic Hunter. Even when I am mainly out hunting for older coins and trade tokens when hunting ghost towns, homesteads, any plowed fields or rough, challenging terrain, I use this approach, and it is very simple. I run the highest Sensitivity I can, and the least Discrimination I can tolerate.
Usually I use no more than ONLY iron nail rejection, just barely, even if just a bit 'ticky', and I recover all likely targets. If I have Tone ID that provides a 2-tone audio, one low tone for iron or just for targets below a low Disc. setting, and a higher tone for all accepted targets, then I might use no discrimination other than audio-based rejection. I will glance at the visual display and get an idea of the TID or VDI read-out, but it's just a glancing reference. It's not used to make a dig/no-dig decision. If I get a good beep, and it doesn't sound like a large target, then I recover it and let my eyes do the most accurate discrimination. That's the only way.
I mention this because when I am hunting a private yard that is well manicured, I know that most Indian Head cents and many early 'wheat-back' cents from 1909 to about the 1920's, read lower. They are very similar to the modern zinc cent. But if I am hunting any site where I don't have to be quite as careful with target recovery as in a manicured private yard or park, then I want all possible good targets and challenging ground conditions (rocks, pea gravel, dirt clods, etc.) can also cause some targets to real lower. Sometimes they might read as iron. On occasion you can get a higher reading, but the fact is that everything isn't always a nice-and-proper reading from our detector's visual source.
And since you also mentioned the US nickel coin, I have pulled them ranging from 48 to 61 with my T2 over the past 3 months. The cause? I was working some renovation dirt with a lot of lumpy dirt clods and some mixed-in smaller rocks. Dimes, even the silver ones, usually read anywhere in the screw cap/zinc cent range, and none of the recovered coins were deeper than maybe 3