One of my professors was a biologist before he became an animator, one thing he said was that art was harder then science.
What he meant by this is that in science there are rules, you have to follow them because that is how the universe works. While in art there also are rules, you can and should break them.
In art the rules are like training wheels, they keep you on the straight and narrow but can be limiting. Some of the best books I have read broke one or more of the rules for writing a novel. The important thing is that they did it for a good reason.
The best way to think about it is that breaking rules is an evil, which you have to do sometimes to do something important. If your going to do something like start your novel with a flashback, then you better damn well have a good reason to do it. Common wisdom became that way for a reason, its best to follow it without a good reason to deviate.