Hi Simon, there is a view rotation mode that keeps z always up all the time.

That's under Options > View > Rotate/Pan/Zoom options > Rotation style = Rotate around world z axis.

If you like having Z pointing up then the usual way would be to have it set to "Rotation style = Rotate around world z axis" and not "Rotation style = Free rotation" which does not keep z pointing up.

If you want to have "Rotation style = Free rotation" and push a shortcut key like Z to set z-up occasionally then that's possible by setting this up on a shortcut key (Options > Shortcut keys):
script:var v = moi.ui.mainWindow.viewpanel.getViewport('3D'); v.tiltAngle = 0;

- Michael