Unity's recommended way to create the tiles is to import a png and then slice it to create a sprite set. The slicer can skip fully transparent tiles and that seems to the the reasonable thing to do ...

YuME is a map editor extension that allows for the rapid creation of game levels using a set of 3D 'tile' prefabs. YuME has been on the Unity Asset Store for 6 years - and it's time to be open-sourced ...