Another reason for not leaving a live round in the chamber is that, if the weapon is involved in a fire, the round in the chamber may "cook off" and the projectile exit the muzzle with the same velocity as if the trigger had been pulled, endangering emergency responders and onlookers. Rounds in magazines or loose ammo may also rupture or detonate, but it has been proven that the fragments produced usually will not penetrate firefighter protective clothing.
Having said that, if you are carrying a firearm on your body for personal protection, have it ready to go, without need for racking the slide, etc., which is one more procedure you can screw up under stress. It might even be said in court, "Well, you had time to put a round in the chamber before you fired" which might look like premeditation. Remember, juries are often composed of non-gun knowledgeable folks and are easily swayed by so-called "experts."
Or do like I do-carry a revolver (I'm old school that way)
I'm just saying that stored firearms might be safer with a round not chambered in case of fire-if somebody is breaking in, you probably will have time to charge the weapon for action. My shotgun is stored with rounds in both chambers, but it is in a vertical bracket in the closet, so if it cooks off in a fire, the buckshot is going up in a relatively safe direction. That would be different if I had the twice-barreled shootsgun shoved under the mattress or in the over-bed cabinet.