There are tons of different ways to practice mania. You should pick what level of difficulty you want to practice yourself. It depends on what kind of improvement you want.
Playing easier maps (for example, improving low S's to 98/99's, or A's to S's) will improve your accuracy on easy maps. It will not really help you with anything else, however. If you want to far SS's or something, that's how you practice.
If you practice maps that you can get high B's or low A's on, you'll begin to improve at hitting those patterns better, and also get some improvement in accuracy on easier stuff.
If you practice maps that you actually struggle to pass, or get low accuracy scores (less than 85%), the things you improve on do not have big effects on your accuracy at any difficulty level, but will help you eventually improve at maps that you struggle to pass on in general. What I mean is that no matter what your skill level, you'll be able to spam through hard stuff better than some players of equal skill level on easier stuff. This kind of practice effectively pushes your overall skill ceiling up, and increases the difficulty range of things you can pass, but is much slower at improving any other difficulty level scores.
Keep in mind that the difficulty of what you practice is only part of things. The other important thing to consider is what patterns those maps contain. As you begin to play harder and harder maps, you will encounter more diverse kinds of patterns. Some of these patterns take a lot of practice to handle, or require different skill sets than what you usually develop as you slowly improve (inverse LN patterns, complex LNs with unusual release timing, chordmashing, chordjacks, etc. etc.). Once you start finding these kind of maps, you might want to practice specific kinds of patterns more than a specific difficulty.
You might also want to consider practicing based on what your actual skills are. For example, I've been currently practicing based on my difficulty with reading. Once you learn what skills you are strong or weak in, you can specifically focus on trying to improve at those skills by picking maps that test those specific skills.
Basically: Pick what you want to improve on, and find out how to practice for that specific thing.