There is no reason to let the hammer fall with the frizzen open. Doing so will eventually break the cock's head clean off...the inertia of the top of the cock not having already struck the frizzen with the flint, will tend to keep the top end of the cock falling forward, even after it stops on the bridle and or the plate. It's not designed to take that kind of abuse, and is easily avoided. Why in the world would you ever trip the sear at full cock with the frizzen open?
And while I'm on the soap box, here's another DON'T DO IT! Never snap set triggers (dry fire) when the lock is in half-cock position. The trigger's mainspring has enough power so that when the hammer trigger strikes the sear arm, the tip of the sear can be sheared off, and on occasion, the half cock notch of the tumbler can also be sheared off. I had a rifle on my bench last week whose half cock notch was broken off the tumbler by doing this. And it's the second tumbler this fellow has destroyed doing the same thing. I explained it to the client (again) and this time, I think he got it. If you want to test the triggers for weight or crispness, leave the hammer or cock at rest in the down position without a cap on the nipple, or in the case of a flintlock, cock at rest down and frizzen open.