Probably z pointing upwards is more common.
https://en.wikipedia.org/wiki/Three-dimensional_space
https://en.wikipedia.org/wiki/Cartesian_coordinate_system#Th...
https://www.google.com/search?udm=2&q=3d+coordinate+system
I see, fascinatingly inconsistent :D I somehow have the urge to popularise a coordinate system where the x axis represents elevation now.