About using a metronome when studying a piece with tempo variations
Using the metronome is especially useful when studying Beethoven, Chopin, Liszt or Scriabine. You should be able to play them in straight tempo, and the metronome can help you do that. When this is achieved, you can use it to train for a given acceleration or deceleration.
It is often tempting but dangerous to begin applying too soon most expression and tempo variation indications. When you do that while you are still working out technical points, overall speed or memorizing the piece, you end up freezing these interpretations together with the other learning processes and you loose some ability to hear yourself.
The risk is that an uneven, excessive or unconvincing acceleration will stick to your memory and you will have a hard time polishing it. You will have also lost a certain perspective needed to really interpret the piece instead of playing it.
Short ritenutos at the end of parts or subparts are often easy to do but continuous acceleration over more than 4 ou 5 bars can be uneven or too concentrated (you can easily run out of acceleration capacity after 2 bars, what do you do next?).
Fortunately one can train for acceleration (as you would train for certain styles of keystrokes or for ornaments)
Using the metronome at one stroke by bar, quite slowly, you can use it to set a target. Say that you have to accelerate over 3 or 4 bars. Try to have a whole bar in advance over the metronome when playing the first beat of the 4th or 5th bar, so that you will play it on a stroke of the metronome. If you have already mastered the piece at steady tempo you won't need to listen to the intermediate bar signals: just ignore them now. By using integer bar time relations like this on longer and longer parts you can build an attention-grabing acceleration.
Before you train to do that on a real piece, try that on something simpler and more repetitive examples or routines, of various length and rythmic structure. It will help you train for listening to or ignoring the metronome at will.
Traits and small odd groups
There is another aspect where the metronome is a valuable friend : large "traits" with many small notes fitted in a few beats. Chopin's music contains also little groups of 5, 7, 11, 13, ... notes who must fit a few beats. You can treat them as local accelerations (more rarely decelerations). Metronome test for this passages is especially important because most players slow their left hand to accommodate the extra notes instead of accelerating the right hand (which usually plays the trait).