Many people will tell you, that you should play every note of a musical piece exactly as written. They say, you’re not allowed to add notes or revoice chords — you should not „mess“ with a masterpiece. But is that really true?
Expecting music to be played exactly as written is in fact a phenomenon of a rather small era in music around the late 19th century. Before that and after that, a lot of written music was meant as a “guide” and the musician was free to interpret it how he sees fit in the current circumstance. Examples of this are the basso continuo in the Baroque or the lead sheets in Jazz and Pop. These styles only give rough ideas of the harmonies played and avoid spelling out every single note. They expect a knowledge of music theory from the musician, so that he can adapt the written guide to the instrumentation, audience and musician’s capabilities.
Classical composers also viewed their works as guidelines. Although they wrote out every note, they avoided to state too much detail about dynamic, agogic and articulation (loudness, tempo and performance technique). Again a knowledge of music theory was expected from the musician to interpret the piece accordingly.
Only during the Romantic era began composers to expect, that every single articulation was played exactly as written. Some of them were obsessed with pinning down every minute detail, so that the music would sound exactly as they intended in every concert. You can immediately recognize late Romantic sheet music by the sheer amount of dynamic, agogic and articulation signs. Remember, this was the era right before „recording music“ was invented. After we had the recording technology, there was less need to cement every exact sound on paper. In fact, today many musicians work from recordings, using lead sheets or sheet music only as a helpful guide.
To answer the initial question: I believe, you don’t have to play music exactly as written. Most pieces were meant to be interpreted by you using your knowledge of music theory. For pieces of the Romantic era you might want to use caution and stick to the written music more closely. Also, if your knowledge of music theory is not that good, you might want to stick to the written sheet music closely. Or you might want to brush up on your music theory skills — check out my YouTube channel on real world music theory for a hands-on way to do that.