Personally... and I say this with much experience returning/exchanging for petty stuff.... just keep it. It's NORMAL. If it gets worse in the next YEAR, send it back/get it exchanged. Otherwise, it's normal.
To be honest I wouldn't doubt it. My iphone 5 had that problem and even Apple said it was shot. All my friends were like "oh no it's not bad" and at Apple they said it wasn't right and replaced my phone.