How sharp a picture do readers have of your characters' appearance? How sharp a picture do readers need? I don't decide this question, my characters do.
I want readers to know my characters from the inside out rather than from the outside in. I want people to know Charlie is always hungry or Suzanne is afraid of heights. Someone's fear of poverty is far more interesting to me than what colour of shoes they have on their feet. However, does this character have a fixation about shoes? If so, I'm going to describe each pair they choose to wear in detail.
I'm certainly not suggesting saying nothing about outward appearances; readers want to be able to develop characters' pictures in their heads. I'll let people know if Mike is so tall he needs to duck through doorways or Cynthia's feet don't touch the floor when she sits. Hair colour? Probably. If they're heavy or slim? Yes, but in an indirect way. I'll describe the character feeling the pull of a shirt that's too tight or admiring their profile in a mirror. If other characters take note of someone's startlingly blue eyes, then that's great. Someone has to wear thick glasses; readers will know about it because of the limitations that character faces. How well do they maneuver without the glasses? Ahh...perhaps he/she can function perfectly well and what does this mean? Or perhaps they can't function at all so what happens when the glasses get broken? Is she/he the type to have a spare pair (or maybe more)?
People can't help the way they look (well, in this day and age they can if they have the money, but my WIP takes place 70 years ago). Some people spend their lives blighted by their hatred of their nose; some simply don't give it a second thought. A character may have curly hair; I mention it if she hates it but constantly receives compliments about it. Everyone is initially judged by external appearance, but the story lies in people's internal workings.
Do you ensure readers have perfect photographs of your characters? Is it important? Do you have a different view when you're the reader? For me, inside wins over outside every single time.