It depends on the beer really...I mean if I'm planning to do a beer that's supposed to come out at 1.082 and it comes out 1.060, and I know that it will ferment down to 6.5% ABV anyway, then I might just accept the beer for what it is and move on. If I'm supposed to get 1.060 and I hit 1.030, I would rather reduce the volume and bump the gravity up. Also, it depends on the flavor of the beer too. If I need to add some DME/LME to bump the gravity, but it's by a huge margin, I'd rather just boil off some more and meet it half way instead of relying completely on a boil or extract. Extract makes really good beers, don't get me wrong, but if I'm doing all grain I don't want half of my fermentable sugar to come from extract unless it's an absolute emergency, so I would rather boil it down some. A really good practice is to shoot for greater volume at less efficiency, though. Make a batch for 5.5 or 6 gallons instead of 5. Take your previous efficiency and reduce it by 5 or 10%. As you get better, close your margin.