When I first bought my iPhone 6s Plus on November one week later of use I started to realize some white spots on the back of my phone. I was using a clear TPU case by the way. I went to a Apple Store and they said that it was dust inside of the case and they have scratched the aluminum of the phone. I couldn't do anything. I was really upset because it's not a cheap phone. But, ok. I started cleaning the dust in the case weekly in order not to scratch that. Guess what, the spots started growing out of control. It looked really disgusting. Awful. So I went to a different Apple Store and they took my iPhone to analyze. I was with no phone during three days then they call me saying those spots were aluminum discoloration. Something really rare in a iPhone and thank god they gave me a new one.
If dust scratches you phone? Maybe. But if you wanna make sure your phone is completely intact take of the case and clean it weekly. This is what I do seeing what happened before I'm afraid now.
So I clean the case and the back of the phone weekly. I don't think you need to buy specific cases for you phone nor Apple cases. Just take care.