I'm not sure if it's possible to know what's true per say, but there's good evidence for manumission being common in Rome. Here's a debate on Roman Slavery and the Rate of Manumission. The Roman Republic and Empire were around for a long time, therefor it depends on the time frame. Example: Apparently, freeing slaves in an individual's will had become so widespread in the late Republic that in 2 BCE the Lex Fufio-Caninia was passed limiting the number of slaves a citizen could free in their wills.
