I think you'll certainly enjoy it. No you won't need to know anything about the subject matter. The film itself will tell you what you need to know and it works on it's own merits without you needing any additional information.
I didn't watch this when it first came out but since i started listening to Johnny Cash's music i gave it another try and i Loved it!
My opinion is probably a bit bias though seen as i am a fan of Cash before watching it.
Funny thing is even though Phoenix and Witherspoon are great in it, truly great, it is actually the scene from his childhood that made the film for me. When he is a small child watching his brother die and asking them to do something. Got a lump in my throat watching that bit.