Shading is a stylistic choice. Some comics have it and some just have flat colors.
No shading like
Isaac At Life
One tone shading like Postcards in Braille. Notice that each color has 1 tone of shading on it.
Two tone shading like Torakun Comics. It's like 1 tone but with highlights also added in.
One step above that like Sanity Circus. The shading is still simple shapes, however, now we're getting into rim light and more complex shading/highlights.
Full on digital painting like
Heart of Keol. Even with this style, the artist picks the panels they go all out on and simplify the shading on panels that aren't as important.
Black and White comics like, my series Oops, require more contrast and black placements.
It's important to have strong silhouettes and a good grasp of foreground, mid ground, background with this style.
Grayscaling also requires a strong understanding of contrast so the page doesn't bleed together into a grey mess.
Chris Wharton of
Giants! know what I'm talkin' about. Look at that strong silhouette, black placement, and line hatching.
Or Megan Kearney's Beauty and The Beast has this concept mastered. The background is super dark while the characters stand out in white.
In conclusion, there is no right or wrong amount of shading to use in a comic. It's your style choice to choose how much to add, do what feels right. Just make sure your shading is correct. Shading turns bad when it's not consistent to the light source or if it muddy ups the scene. Use it wisely.