What’s the best way to approach the opening, to learn exact moves or rely on principles and common sense? Actually the question should probably be rephrased so as to get a better answer; everyone needs to have a good grasp of opening principles whilst at higher levels the preparation of specific lines becomes increasingly important.
What about amateurs and juniors, do they need to know some book openings? Well given that their games tend to leave theoretical lines after just a few moves the answer has to be ‘no’, and very often there’s still a great need to acquire a deeper appreciation of the ideas.
In the following game we see Black’s violation of opening principles drastically punished. Did a top Grandmaster like Efim Geller really not know that 8…Qf6 is a dubious looking move? I’m sure that he did but he probably thought he could get away with it. And it needed Fischer’s superhuman play to show otherwise: