This is a confusing issue. With centerfire ammo Ruger recommends dry firing their revolver to get used to the pull. I think Sccy does as well but I can't recall for sure.
Almost all modern centerfire firearms out on the market can be dry fired without a problem.
If you spend all day practicing, you might eventually break or wear out the firing pin. We're likely talking about thousands of trigger pulls here. A new titanium firing pin (likely of better construction than the old one) for a 1911 might cost $20 or so bucks. For practicing, that's still cheaper than a box of ammunition.
A few older centerfire designs can not and should not be dry fired (even a few times). The CZ 52 instantly comes to mind (and yes they do make snap caps for 7.62 Tokarev).
Would I make it a habit of dry firing all day without snap caps? The answer is simple - no. A set of snap caps is cheaper than a firing pin. If you can't afford the snap caps, then you can't be buying ammunition in the first place.
You should really buy a set of snap caps if you want to incorporate dry firing into your training regiment, however pulling the trigger a couple of times isn't going to damage anything (excluding firearms like the CZ 52). Glocks for example require you to dry fire the weapon before a field strip.
So, occasionally on a modern gun? Absolutely fine.
This is one reason gun stores sometimes have a policy of asking you to not dry fire the floor models. That high dollar Kimber might be handled by five or six people a day at a busy gun store and dry fired five or six times by each person. That would be over 6,500 trigger pulls a year.
If parts and/or the firing pin and hard to obtain for your particular firearm (out of production), I certainly wouldn't make it a habit. Even a firing pin for a older, less common gun such as the Star Model B is only $25.
In all honesty, that derringer is not built well. Dry firing the derringer with the safety on will damage the pin in quick order. Snap caps should be okay. I think the one individual I located on the internet who damaged his derringer was using a snap cap in which the fake primer insert was missing (the same as firing on an empty chamber). The CS for Bond Arms seems to be pretty good so if something does happen with snap caps, I'm sure they will remedy it.
I'll end with this. I've read that Glock on multiple occasions has claimed that excessive dry firing can cause the breach face to crack. Regardless of whether this is true or not (some believe that the breach face cracking problem on a few Glock models is a metallurgical issue), isn't a few bucks on snap caps a cheap way of telling CS I've always practiced/dry fired with snap caps?