I am always using symmetry...
I am making Left wing/flaps/gear doors...reset pivot point and set it to X-0.0 as the center of aircraft/symmetry line. Then resetXform. And then modifier - symmetry. Mirror axis- x and If I made left-wing then check 'flip', if right-wing - symmetry will appear on the left side..Still in most cases, i just need to reposition the pivot point on X- axis. But it is way better than Mirror, no need to resetpivots/xform/flip polys This is my way...I think it is quite easy,
I use symmetry almost to everything...If I make landing gear..when gear is finished...then I am UVmapping...and making symmetry. - Then I can sort parts/animate etc... But i have symmetrical and already mapped elements.