Pst of basic (but somehow also, seemingly, top-secret) info from Bret Pimentel, Use your metronome most of the time, with some excellent quotes:

I know very few musicians who have the problem that their tempos are too steady. It’s important to practice the tempo nuances too, but if you can’t play the line in perfect time then you probably can’t do a convincing accelerando/ritardando.

In my experience, there are two kinds of musicians who think they don’t need a metronome. One is the top 1%, who have spent a lifetime developing world-class musical abilities. The other is beginning and intermediate musicians, who haven’t learned the value of metronome work because they haven’t done it enough. Don’t mistake a top-level musician’s musings for good beginner advice.