By a metallurgical engineer from one of the frequent thread on Trapshooter.com
All ferrous metals, like springs, will "take a set" over time, and all ferrous metals. like springs, wear due to being "cycled."
However, the "take a set" takes lots of time, many years, and is enhanced if the metal is at high temperature (which gun springs, generally are not.)
So, you have to ask yourself, is it more damaging to the spring to "cycle it" one more time to de-tension it (and remember, it is STILL under tension even if you let the hammer down) or is it more damaging to leave it under tension.
For gun springs, especially if you replace them periodically like you should, I am pretty sure the answer is, it doesn't make a darn bit of difference.
On the same thread it was stated that both Beretta and Perazzi, and made in USA Kolar, recommend using snap caps.
Another engineer on the same thread
As an engineer I look at the Perazzi spring and question the extremely small radius at the corner. The stress concentration must be very high near the corner. I also wonder how the pin is attached. I think that it is welded which can do interesting things locally to the spring metal. Sometimes that local heat can help relieve manufacturing stress, sometimes it can really screw things up. Again, without knowing what manufacturing processes are used, I would just think that increasing that bend radius would greatly expand the useful life of the spring. Marcello Guliani's version of the spring must have a longer working life because of the way he rounded that corner. Once I run out of factory Perazzi springs I will switch brands.
What we do know is that it is very easy to either drop the hammers on a snap cap or a penny, or pull the trigger assy out and drop the hammers manually. Why would you even risk shortening the spring life by leaving them cocked?
For those with coil springs, they don't seem to be as sensitive to breakage, but again, why shorten the life of the spring when you can drop the hammers so easily?