I always let go of the reins if I fall off.
Most of the time it's natural, I just fall off & let go. It's like automatic, LOL!
My horse never runs off, she always stops & waits for me to get back up, ahaa. Even if we're hacking out & if she gets spooked or something she never runs off, even if she was cantering away, she always stops. :)
But my friends horse, if she falls off she holds on...I dis-agree with it but it's only because her horse NEVER stops. So I guess it's better in her situation.
But no, in XC maybe you should hold on but still my horse would never run off so I'm good, LOL!