Matt,
I imagine any modern
circulating coin must have the value included on the coin. There were a series of 25p coins from the UK in the 1970s that did not include the value. Because of the size of these coins, I have to assume they did not circulate.
https://en.numista.com/catalogue/pieces5488.html
You're right, though...not including the value would lead to confusion or worse, as happened with the United States 1883 nickel (5 cents) where the US Mint left off the word CENTS. The value was simply a roman numeral V on the reverse with Liberty on the obverse, which led to gold-plating of the nickel to pass it off as 5 dollar coin.
http://www.coinsite.com/content/faq/RacketeerNickel.asp
Back when the value of a coin was tied directly to the value of the metal in the coin, there are other examples of coins without the value included. There are numerous 1/2 Real coins from the 18th century that do not include a value, but based on the size compared to the 1 Real coin (which did include the value), it was obvious to everyone what the value was.
David