Every music major has to take music core. We take two years of music theory, dictation (writing out the music that we hear), and sight singing (so we can hear the music we play just by looking at the score). In music theory, the main focus was learning the rules and principles of correct part-writing. This focused mostly on hymn-like or hymn looking music. We learned the rules of voice leading, part writing, and harmony. We were graded on how well we followed the rules we were given. However, we always found exceptions to the rule in different musical passages. When we brought it to our teacher, he told us that it was okay to break the rules. We were learning them, so then we can break them judiciously.
I think it's the same in piano playing. As a student, I need to be able to follow all the directions provided in the music explicitly. Anyone can decide not to follow the instructions or to change things on their own, but not everyone can do everything written in the music. Once you can follow all the directions, then you can take your liberties as the musician and change things according to your knowledge of the time period and the composer and the piece.
Basically, the more you know and the more you're able to follow the directions to a T, the more space and room you have to be creative to make music the most beautiful it can possibly be.
