Hi L., I think there are other reasons kids develop habits, or crutches, "isms", besides just comforting, or nervous habits.
So the question of whether or not a parent should take steps to stop it depends on how socially unacceptable the habit is, you know?
Here's an example:
When my daughter was about 3 months old, while nursing, she would reach up and rub my eyebrows, she was OBSESSED with my eyebrows, which sounds sweet but got a little annoying. So I would take her had off my eyebrow and put it on HER eyebrow. In hindsight, I probably shouldn't have done so, since she soon became content with rubbing her own eyebrows every nursing session.
I don't believe she did it to sooth, as the nursing itself was soothing, I believe she did it to FOCUS on the task at hand. Which to begin with was eating.
Her right hand steals up to rub that eyebrow ever since while reading, eating doing homework, using her phone, watching a movie, these are not times she needs comfort, but times she needs to focus.
I have pointed it out to her through various points in her life, but it's not gross like sucking your finger or biting your nails, and it's not a constant thing either. At almost 17, she only does it now while alone, or at home with family members.
I'm not sure what I'd have done if I'd had a thumb sucker or a nail biter. So it's easy for me to say well you gotta get rid of that now, or aaaw just let them be.
My boys were both shirt chewers in k through about 2nd grade. And that WAS gross, plus too much to keep replacing shirts. I suppose I shamed them about it. Or maybe a kid in school teased them about it, at any rate they both stopped in 2nd grade and didn't replace the habit with anything else.
I think every child is different, and each may or may not need one of these "isms" for comfort or focusing or whatever, so it's impossible for there to be only one rule deciding what to do about it. Also, with some kids, bringing it to their attention actually makes the urge to do it worse. Or if they stop, they pick up some even worse habit.
But yeah, seeing an 8 yo with his thumb in his mouth at the grocery store with his mom makes us cringe, and judge too. When we don't really know the whole story either.
I have three kids. They all have different coping mechanisms. They've all been making their way successfully. Most people do, regardless of their childhood crutches.
Great question!
:)