Funny this topic comes up... just a couple of weeks ago, I tasted a cider I made 12 months ago that had an OG=1.042, and my tasting notes were that it was kind of weak and beginning to taste stale. Meanwhile other ciders I made that were in the upper 1.040s are great. As such I decided that if my OG is ever less than 1.045, that I will add just a very small amount of sugar to bring it up to the upper 1.040s.
Other than that, I am really an advocate for not adding sugar at all. If you can find juice made from really ripe apples, OG should be closer to 1.048 or even above 1.050 in some cases. Then I certainly would not add any sugar. What I don't like is when people say "I added 3 pounds of brown sugar just for the heck of it, plus a couple cinnamon sticks..." To me, that ain't a great cider anymore. If you add a few ounces, fine. If you like wine, fine. I guess I'm more of a purist. But at the same time, I do treasure shelf life and will make very small tweaks in this regard.
If you plan to drink all your cider within 6 months, then I wouldn't add any sugar either. But it's true, the higher gravity will get you more alcohol which will preserve it better. Just be reasonable about how much you add if any.
blah blah blah... yeah, I write a lot.