In my opinion (and you know how opinions go, there is an old adage that everybody has an opinion and they usually all smell like someone else's armpits) I know the older wheat pennies can read within a notch or two of modern clad dimes or quarters. I think it is because of the copper content contained in modern clad dimes and quarters, versus the older (pre-1965) silver coins. For example, a post 1983 penny is approximately 97 % zinc with a copper plating, whereas an pre-1983 penny is 97% copper with zinc alloy. I don't know what the current composition is of modern clad, but I do know the older pennies contain more copper, versus the lesser copper content in than of the pre-1965 "silver" coins, which are approximately 90% silver the rest are other alloys. Espcially when you get more than one penny, or even pennies and dimes mixed in the same hole, they can cause a higher reading as previously stated by Goes4Ever (and he has some awesome posts!). I think this is why the x-terra mistakes the older pennies for dimes and quarters. The machine is making an "estimate" of what it thinks the metal in the ground is, based on composition, conductivity and size.
As indicated by previous posts, it also can depend on the type of ground conditions you are hunting in. The conditions and resulting signals vary from gravel (which can contian high mineralization) sand, wood chips, loamy soil, and wet/dry sand and/or salty beach type conditions, and whether the coin is underwater or not.
In my experience, the older the pennies, the higher reading you will get equating a dime or a quarter, and then again it depends on soil/moisture conditions. As mentioned above, the best thing to do is "DIG" if you are in doubt. This is always the best "discrimination" setting you can use. As stated above, the numbers may "bounce" around a little as the coin or item may not be laying in an ideal flat position, but may be tilted on "edge" or located on top, underneath or adjacent to items such as foil, rusty nails, bottle caps, etc, which can cause the jumpy numbers also.
A "pocket spill" of coins, a chain, necklaces, bracelets, pendants, bottle caps, soda can pull taps, beaver tail style pull rings, gum wrappers, wads of aluminum foil, or even the small foil seals from juice containers have caused my x-terra numbers to jump around. And last, but not least, there are gold rings I have recovered which read in the same range as the foil, pull tabs, pennies, dimes and nickels. I have found that when I have those conditons where the numbers are "iffy", if I scan very slowly, and do a scan from different approaches, I can verify a consistent signal and then I dig. If not, I'll note the location and come back and dig at a later date when I have more time it I don't want to dig right away. Occasionally I mistake a quarter, or a multiple quarter pocket spill, for a piece of a buried aluminum can even a solid piece of aluminum scrap. I pack that stuff up and go on my way. As Red Green likes to say "Remember, we're all in this together". I don't penny bomb anyone one the beaches or anywhere else, and I don't bury my false hits back in the ground to slow anyone else down. WOW~~ :You guys that can hunt in the end of those upper end sensitivity ranges have my respect. I am out in the Pacific NW, because of the gravel/mineralization (and mostly because of the glacial deposits) the coins I find aren't more than 3-4 inches before they come to rest on the hard packed glacial deposits. I don't remember ever having to go above a 15 sesitivity level. And so I don't usually have to dig more than just a couple of inches, which suits me just fine! I hope this advice has been helpful to all of you. Good luck, and happy hunting my friends.
However, after hearing the weather guesser calling for 5 - 7 inches of snow tomorrow, I doubt you'd want to be in this corner. 
Anyway, over here on this side, my ground phase is nearly always in the 40's or higher. That is why I don't waste time digging targets that fail to meet those three rules of consistency. Location, audio and display. Next spring, when the ground thaws out, we'll get together somewhere between the rivers, and see what we can find. JMHO HH Randy