From CNN:
You might have heard this for years now, but the penny and the nickel cost more than 1 & 5 cents to mint respectively. The dime is not too far off, but is for now safe. There is a nice graphic in the second article that helps one visualise this.
Obviously, it's a little deceptive, because each coin is meant to change hands many times over, carrying its' face value's worth of GDP each time.
A more pertinent question would be, what is the cheapest item worth in your economy? Those are typically boxes of matches, lighters and Chupa Chops. Here in Australia, those used to be 1 cent, then 5 then 10 and now 50. This doesn't render lower denominations useless, but as inflation moves forward, lower-value coins become impractical.
So watch out for those last rolls of US pennies and Nickels. After a few more years, there will be no more.