Monviso The notion that the net balance of the "ebb and flow" should be zero is an interesting reminder that fluctuations go in both directions: both faster and slower.
People say this but I would say reality is much more complicated. There are places where you just accelerate, and places where you just slow down. I think the only real way, unfortunately, is to listen and accurately imitate. It's like an entire rhythmic language unto itself. And you can not reduce a language to a small set of logical rules.
I basically never use the metronome. My teachers also have not recommended that I use it, asking me to develop my internal sense of pulse instead. To this I credit developing a better, more fluid and "real" sense of rhythm. But it might not be for everyone. It is really part of the current zeitgeist but when I look at books and recordings of the old masters especially, I seldom see the use of the metronome so advocated.
But you need to develop a sense of rhythm. I can play in time with a metronome if I wanted to, and that is important.