My simple process to set ‘bars and shifters:
1) Handlebars loosely installed in stem. Shifters loosely fitted but largely ignored at this point. Sit on bike (against wall or with feet on something), close your eyes and reach for the handlebars. Do the hooks feel in the right position? If the top section is shaped, does that feel right? Adjust as necessary and nip up the stem bolts.
2) Shifters roughly in place yet loose enough to move. Sit on bike (against wall or with feet on something), close your eyes and reach for the shifters’ tops. Relax and assess if they FEEL right. Move them if not (vertically and rotationally). Nip them up.
3) Go for a short ride; you should open your eyes now 😉 Does it all feel ok? Tweak if not. Torque it all up, then apply tape.
The shifters may well not be perfectly symmetrical, but neither are you. Nor are the roads ever 100% flat. Comfort and feel should almost always trump aesthetics 🙂