The ones I have used will not open until you have cocked the hammers after firing. On mine the firing pin will not allow the gun to open. I have not used that many of them but most are a short cock to fire the gun which is similar to taking off a safety when talking about the time element.
The geometry of my non-rebounding Lang is such that it will open and close with the hammers down. The issue is that if you leave the hammers down and slam it shut it could fire.
The procedure is simple - fire, half-cock, open, extract, reload, close, cock, fire. After you use a non-rebounding hammer gun for a short while the procedure becomes automatic.
I prefer non-rebounding hammers but I also own a 24 gauge with rebounding hammers. I like all guns.