I used to be one of those musicians who looked for rules that I could use to make my playing better. Since I was a flutist in my youth, baroque music made up most of my daily practicing, so the rules I tried to apply to my playing often came in the form of method books, many of which were written in the 18th century to dictate the idiosyncrasies of a particular musical style or to reflect an individual musician's idea of how to play musical phrases the way he (yes, all the treatise writers went by "he" during the baroque period) thought they should be played.
I still come across young people who like to apply "rules" to the interpretation of baroque music. It used to bother me to witness their insistence on a particular kind of musical behavior, but now I just kind of smile to myself and hope that these people will, one day, understand the applying any kind of across-the-board rule to any kind of musical interpretation is just part of the experience of a growing musician. Like other maladies of youth, it too will pass.
Not all rules are bad. Actually, there are a lot of technical rules that can be applied quite successfully in order to build an instrumental or vocal technique. On the violin these rules concern things like shifting properly (actually I think there is only one way of shifting properly and there are a lot of ways of shifting improperly, but this thought might be a result of my relative violinistic youth), counting beats accurately, and keeping fingers down during string crossings.
I cannot think of an interpretive musical rule I have learned that applies universally. There are no hard and fast rules concerning trills (and these are the first rules that people look for) because there are always exceptions. There are no rules regarding articulations, because music has all kinds of articulations, and they are used for all kinds of different reasons. A particular articulation that is well suited to the oboe may not be well suited to the violin. Therefore an oboe player would be more likely to write a treatise that favored oboe articulations to the kinds of articulations that are used in writing for the violin.
There are no hard and fast phrasing rules (for music of any period), because every phrase of music (worth playing) is unique and beautiful in its own way. Each phrase is a separate case, and each phrase can be interpreted in a multitude of equally correct ways, depending on the desire(s) and mood(s) of the person who is playing. Applying stress to one beat instead of another can be executed tastefully or non-tastefully. I have come to understand that interpretive musical rules do not guarantee excellent musical taste. They don't even guarantee good musical taste.
Some of the novice rule book players seem to believe that one set of rules applies for all "early" music. A set of early 17th-century French rules would sound kind of odd when applied to music from C.P.E. Bach's Hamburg, or Mozart's Vienna, yet there are people who seem to believe that somewhere there is a set of universal rules that tell the "way" to play all "early" music, no matter where it comes from. I hope that all of these people dig deeply enough in their early music studies to learn the folly of their relative youths and in so doing understand that all musical interpretation is subjective, creative, and personal, rather than uniform and formulaic.