I always have to read these threads, just to see if there are any new answers.
I don't know about BuckMarks, they look like they might be the type that can be field stripped with the hammer left cocked.
However, many rimfire (as well as centerfire) pistols must be de-cocked first in order to field strip. That means, on a good number of models, that you'll have to "dry-fire" them at least as often as clean them.
For this reason, I surmise that occasional dry-firing isn't going to harm anything.
But, most of the rimfire pistol owner's manuals that I have read, have cautions against frequent dry-firing. I think this is becuase the firing "pin" on many rimfires make contact with the face of the chamber and can cause peening when theres no round chambered.
As far as leaving cocked, it would makes sense that a spring left under tension would develop a set. Just think about the suspension in the old jalopy you may have rode around in in highschool. But, again, I suspect that unless you're gonna store it that way for years, its just not going to be a problem.