OK this advice is worth exactly what you paid for it!
I can not think of any technical reason to wait on the evaporator.
IF you were replacing the drier, I can see waiting to install that until you are ready to evacuate the system.
Some factory manual services notes:
...Always immediately plug all open fittings after removal of ac parts... to keep moisture and foreign mat'l out.
I can't find any other significant notes.
HTH