If you have to work on Isometric drawings - you have to design your perspective in at least 6 directions.
QCAD provides a lot of tools like iso grid, move/copy (etc.) with polar coordinates or line from angle.
I'm not doing this every day what means I have always to remember the correct main angle values to work in the desired direction. Ok, maybe that is only me!
However, to make my life easier I designed a "Axis of coordinates" Symbol as a block in my Library and can now use it where ever I need it for my constructions.
Maybe you too.
Just put it into your library - use it in your drawings - delete it (if you like) after your job is done.