Here is a good link that talks about the symptoms and various causes of pink eye and the associated eye crusties. Really, pink eye can be a symptom of many, many eye issues. It can be caused by abrasions of the eye, infections, and allergies...not all of which require immediate medical intervention. Bacterial conjunctivitis must absolutely be treated with antibiotic ointment or drops, viral conjunctivitis will, obviously, not respond to antibiotics.
From my own personal experience with eye infections, true eye infections are really, really painful and cause a lot of light sensitivity. I would think if your child had conjunctivitis, you would see a lot more symptoms than just eye crusties. However, if you notice that she has more than just the eye crusties, they continue throughout the day, or there are other symptoms that she has then she should probably see a doctor.
She might just be experiencing seasonal allergies (I know we've been slammed with pollen this past week here in the southeast), but what starts with just eye gunk can progress to full-blown sinus infection, so, if it were me, I'd keep an eye on it and make sure it doesn't progress any farther.