I was a fencer in high school, not an expert but the only time I showed my back was when we reset each match. Oh and once I was able to lunge low enough to where I caught my opponent on the knee. My back was exposed enough to where he able to bring down his epee on my back. I felt that for a week.
Depends on the situation and armour sometimes it works but you must avoid it as much as possible cause you create openings. For example if your opponent is stronger but you exhaust him enough while you're good and need to make up for the difference of power then yes it works cause your opponent is gonna be slower than you and he can't intercept you
