"I never commit to memory anything that can easily be looked up in a book."
- Albert Einstein -
Everything is depends on your target. If you just wanna play as long you know what the next best move is, then practice make perfect :) But if you want to be a good player (maybe GM) with high rating, you should learn the opening theory.
As for me is everything that suit me, as long i enjoy it.
The night before my second OTB tournament, I became obsessed with the idea that I had no preparation for 1 d4. I stayed up half the night trying to learn the Orthodox QGD. I got to the tournament, bleary-eyed, and had Black for Round 1. The guy played: 1 f4
I learned a valuable lesson: sleep is better than booking up the night before you play. By the night before play, you know what you know. Go with it. Also, no matter what you do, someone will play something new to you, and you are going to have to play chess, and figure out what is going on.