Romantic music (the style) and especially Chopin are known for the extensive use of rubato (however, Alkan didn't like it at all and suggested that even Chopin should be played the classical way but he was quite alone with that at his time and even today). Well, rubato is an extra tool for expression and the best way, to learn when to use rubato is to try it out. Humming alone can help a lot, our voice is the most expressive musical instrument on earth and it has a lot of automatic "features".
I have to disagree with the statement that the music of Chopin is romantic with extensive use of rubato based on Chopin's own statements and those of his pupils. Chopin was appalled that some of his students and other pianists played his works in such an abhorrent manner. He condemned this fancy-of-the-times, among others, that was so prevalent by certain pianists (e.g. Liszt).
Chopin's compositions do not sound good with "rubaaatooo" because:
The forms of his music are based on classical forms. The sonatas follow sonata form and the minatures usually follow ternary form. The reason why these forms developed and why they are still in use today is because it provides a musical struture that gives balance. Chopin's music has very clear forms that allow the other musical elements, most notably melody, to create a balance.
Now imagine what it would be like to apply "rubato" to music that cannot sustain such a device. Imagine the sonatas of Mozart, Haydn, Clementi, or Beethoven performed in such a manner. You probably have to imagine it because all good musicians never use such a device in these kinds of works. Because all the aspects of the music - motive, phrase, expression, form, etc. - require good pacing of the musical elements to be effective, distorting the aspect of time will destroy the structure of the piece by disrupting the balance of the musical elements. This applies to Chopin's music as well which lead to another important point.
Chopin's music isn't romantic at all. All of his music is extremely well balanced to the ear. It's not lopsided like those of Liszt's so let's survey Liszt's. Some of Liszt's compositions contain musical ideas that do not naturally balance themselves. Take any of the Hungary rhapsodies or the Years of Pilgrimage whose musical ideas tend to tip the balance scale one way or the other but doesn't contain a counter balance. In order to balance the piece, and this is
the reason a musician would "use" rubato:
The use of such a device allows a performer to create a sense of aesthetic balance that does not distort the musical idea. Rather, this device creates the framework for the musical ideas to stand upon.
Why is Chopin's music considered
romantic? His music was never considered romantic until his contemporaries used this term, which was originally used to describe paintings of emotional content, to his music. But even before this, his music was considered modern, in the same sense that we call the musik of Stockhausen, Cage, Rzewski, Lutoslawski, et al, modern. In fact, many of the slightly older generation couldn't understand Chopin's music at all. For the older generation, the moderness came from the use of very innovative harmonies and melody-driven compositions. Contrast this to the music that was written and it's quite revolutionary, and Chopin was unique in this sense. The extensive use of long, flowing melodies, which dominates his compositions, to propel the music forward is perhaps the biggest reason many people consider his music "romantic": It sounds romantic.
Or course, what made Chopin even possible at all was the modern piano (modern in his time, not ours). The figurations possible on the piano during his time was not possible on previous pianos. As a result, musical accompaniments could be as original or old-fashion as the action of the the piano allowed and Chopin made great use of the flexibility offered. This novelty, made such an impact on ears that they were to be copied by many other pianist-composers, like Liszt. These new figurations also made a perceptual divide upon the ears separating what we now call the
classical style and the
romantic style.
Even if we were to continue to label Chopin's music as
romantic, this does not justify the use of unnecessary rubato. His compositions are already well-balanced pieces of art that doesn't need any extra help.
Chopin's music is like an exquisite gourmet; the chef has prepared everything to be perfectly balanced and palatable so you don't have to add seasoning to enhance the flavour.
Liszt's music is like a hotdog: the more ketchup, mustard, onions, and relish (and bells, whistles, the kitchen sink) you put on top of it, the better it tastes.
Topic: when is the use of rubato desirable?