Yeah, I agree - observation is the key. It all depends on how well the snake secures the mouse when it strikes. Most times, the mouse is fairly well immobolised, but if they grab the mouse awkwardly, then there is the opportunity for the mouse to bite.
I'd probably be more concerned when feeding a rat - they are far more capable of inflicting serious damage.
There are times when you have no choice. For example, my diamond is going through an awkward phase right now, and won't touch dead prey - she'll will only take a live mouse. So I plan to offer a thawed or freshly killed mouse each feed time. If she takes it, then great, otherwise I'll offer live until the next feed when I'll try again.
... that's my theory, anyway