From Writing funny stuff on ammo is over 2000 years old:

Know those great photos of World War II crews painting bombs with messages like “Easter Eggs for Hitler” or “To Mussolini, with Love”? It turns out, your ancestors have been doing that for over 2,000 years, because the British Museum has sling shot from 300 B.C. where missileers were telling the enemy to “Catch!” their shot.

The British Museum has a good example.

“Catch” is one of the tamer examples. From Humorous Inscriptions on Lead Sling-Bolts (Sling Bullets; Slingshot) Reflect a Roman War of Words:

Evidence of wide-ranging military literacy in the Roman Empire can be of a very ephemeral kind:

“In 41 BC during the civil war that followed the death of Julius Caesar, Octavian (the future Emperor Augustus) trapped Lucius Antonius and Fulvia (the brother and the wife of Mark Antony) within the walls of the central Italian town of Perugia. A number of lead sling-bolts (roughly the size of hazelnuts), manufactured during the seige that followed, have been recovered in Perugia; they bear short inscriptions, which both sides carved into their moulds, so that the bolts [also called sling bullets or slingshot] could be used in a war of words, as well as to inflict death or injury. Some of these inscriptions are fairly tame, wishing victory to one or other side, or commenting on Lucius Antonius’ receding hairline (which is also known from his coinage). Others are rather richer in flavour, like the one, fired from Octavian’s side, which bluntly asks: Lucius Antonius the bald, and Fulvia, show us your arse [L. [uci] A[antoni] calve, Fulvia, culum pan[dite] ]. Whoever composed this refined piece of propaganda and had it cast into a sling-bolt certainly expected some of the soldery on the other side to be able to read”

Also see Catch! Attacking Your Enemy with Words as well as Weapons. See Vodcast 9 FELAS OCTAVI for an even ruder example from the Battle of Perugia.

What these examples shows is that:

1. Literacy was not uncommon among slingers.
2. The expected their enemies to be able read these messages, hence there were literate soldiers on both sides.

Most of these examples are from Roman times. It is important to realize that Roman slingers were auxiliaries. The core of the army was formed from the heavily armed and armored legionaries. In battle the function of slingers and archers was to soften up the enemy before the legionaries went in for the kill. Slingers were more important in siege situations, but they still played second fiddle. Legionaries were recruited from Imperial citizens. Auxiliaries were recruited From peregrini — free non-citizen subject peoples. Auxiliaries were paid somewhere around one third of what legionaries received. If an auxiliary survived his 25 year term of service, he would receive a retirement bonus and Roman citizenship for himself and his families. His sons, if any, could then enlist in the legions.

So it seems that there was a significant degree of literacy among the second-class soldiers. It seems reasonable that legionaries would be at least as literate, especially since the Roman army had a significant bureaucracy with which the legionaries would have to interact.