What would dental schools think when a GPA started out really high, like 3.9, then start declining later on?

There is no definitive answer for this. Generally a downwards trend is not good. Even though classes may be getting harder an applicant is expected to maintain the same performance or improve on themselves.
In my opinion, it ultimately depends on the type of classes (and how many classes) the individual was taking when achieving the high GPA and comparing that to the type (and how many classes) were taken when the lower GPA was achieved. It is important to remember that the application review process is comprehensive. They are not only looking at your GPA but at your workload, extracurricular activities, and any issues you noted on the application like hospitalization.
Ultimately, maintaining the high GPA is ideal and a decline in GPA is expected to make an application less attractive than that with the high GPA. In some cases I honestly do not think it will hinder an application by much, if any.

Posted in: Dental School Admissions