Personally I think there's a lot to be said for moving into the net after you've hit an offensive shot. That having been said, knowing when you're having a lousy day and are not able to play offensively consistently is important as well.
Rafa said it recently at the US Open:
"Better today than the first two rounds. Here we are after six months back playing tennis again in a great position. I am trying to play more aggressively, it's true, but that's because I am playing well. If I'm not playing well, I have to be more defensive."