also, tones are really based on your taste. Its all about value, visual texture and contrast. There's so many style of toning same way there's so many style of coloring.
In general, you should keep your black and white balanced so the heavier you are with your inking and blacks, the darker your tones.
look at clamp's cardcaptor sakura vs X to see the same artists doing 2 different tone styles.
tones can be expressive and create mood, not just laying down shading 'just because'.
I tend to gravitate towards a lot of gradients but many like just flat grays. I don't like laying one gray on top of the other, while many artists do. Its all about balance and looking good so you'll have to practice.