I have both Sigmas and an SD model, and I truly feel both types of guns are safe without a manual safety within the parameters of safe gun handling. But are you talking about keeping it in the nightstand or closet with one in the chamber? Notice that I didn't say whether or not it is safe to keep a gun that isn't on your person loaded in such a manner. That's ultimately up to you to decide based on your own situation. But as far as keeping one in the chamber with minimal risk of it being accidentally discharged by a responsible adult, the SD is safer than most non-manual safety pistols I've run into.
And that's a function of the trigger, so many of the people who buy SD's and immediately put a trigger kit in it, well...I don't understand it. They would've been smarter to buy something like a Walther PPX if they wanted something with a noticeably lighter trigger for target shooting. BTW, I personally don't feel the PPX is a good self defense gun as it comes from the factory. A nice gun, but the trigger is too light for a pistol without a manual safety. Not a function of the trigger pull weight, but the nature of the trigger pull.