It depends on the type site you are hunting. I can think of sites where 99.99999999% of everything that reads "penny"......... will be .........." a penny"! doh! If you look at people over the last 25 yrs. (since TID was invented on the first Teknetics) that have done odds-charts, of where gold rings tend to fall, verses other targets, you will really begin to see the practical implication of your post. For example: If you had the ability to go into any jewelry store, and bench test all their gold rings, and log their coordinates into a computer. Then take a bunch of USA coins, & various re-curring pulltabs, etc..., and log their coordinates into a computer. Then do the computer-odds to see what's the better TID/cross-hairs to dig, you would see that your time would NOT be better-spent digging pennies. Sure, now and then a bigger man's ring *might* read up that high. But it's all in the odds. I mean, SO TOO might you be better off to take a "hit" in black-jack in Las Vegas, when you have 20 in your hand (afterall, the next card *might* be a 1 card, right??)
If you have the time, and type site, that allows you to strip-mine, great. It all depends on the site. I can think of days after beach storm erosion on the beach, where I wish I'd passed pennies. Because I merely got chased out fields of un-told thousands of targets, by the incoming tide. Why the h*ck would I was coins of ANY sort (even old ones that are messed up by salt/tumbling) when I could chase the odd-sounding low conductors, and improve my odds at gold rings? Naturally, those days are rare and far between. But to simply say that someone can dig penny signals and up their odds are rings (I assume you meant gold rings?) does not make sense.
The best bet to getting gold rings at ANY site, is to pick the best possible sites, where rings are more prolific to begin with. Ie.: swimming beaches.