Frankly, I worry less about spring wear here and focus more on the dubious [dangerous/unpredictable] nature of a live round chambered in a cocked shotgun. Now, at the risk of being a bit off topic, would I keep a pistol in this condition? Perhaps...depending of the gun. Sigs and XDs, for instance, make me feel comfortable enough to do so. But, generally speaking, the extra second or less it takes to chamber a round more than makes up for the potential hazards associated with a live round in the chamber. Now, for CC, that is a different story.
Anyway, back to the topic at hand. What I do is the following: I keep 5 rounds in the tube (even though I have a "supposed" 9 round max...in reality, its more like 8) and one dummy round (or snap cap) in the chamber. This dummy round allows me to "safely" drop the hammer and keep the pump in a "disengaged" state. From here, I simply need to actuate the fore end, thus ejecting the dummy round and putting a live round in the chamber. I find this setup much easier on the gun...and much more safe. Essentially, what I prefer is similar to the "cruiser ready" mode used by police.
In addition, I keep my safety on. Why? Well, it makes me feel better. If the sh*t hits the fan, I only need to grab the shotgun, eject the dummy round (thus, inserting a live one) and Im good to go. The safety? Well, when I practice, I only disengage the safety just prior to shooting...during the act of raising the gun to my shoulder (the Mossberg makes this an easy transition, BTW). I try to keep my practice routine [i.e., weapon state] similar to what I would use in an HD situation. For me, this helps maintain a certain consistency (which could save my life one day).