Summary: Do I need to uninstall Rhino for Mac before installing a newer version?
No, you do not. You can overwrite the application bundle - most frequently located in the Applications folder - when you install a newer version. This will not overwrite your license or any of your preferences. Both are stored in other folders.
In fact, you can run two versions of Rhino side-by-side, if you desire. Just rename the application bundle (Rhinoceros.app) to something different (RhinocerosCopy.app - for example).