Mark,
I was painting on both sides during the first months of last year, but:
1) I wished to improve rapidly, meaning spend a lot of time on each flat rather than painting many for large dioramas and painting the 'other side' would doble the time for each piece!
2) I would never expose them sitting on a table, but in a frame, therefore the chance to show the other side is very much close to zero, so why painting always them.
3) They all have one side presenting much better han the other and if eeded (like the hoplire I am exposing, I would paint a second same flat turned on the other side but this is more exceptional
4) Unlike the past, nowadays casting, except with some good producers in 30mm size has a lower quality (experience, aging of the moulds, large amounts instead of high quality for less) at the edges, misalignment of the two moulds, resuting in a lot of filing work; le mislignent of the twi loukls-ds frces you to sacrifice détails on one side to remove the burrs on the other one, therefore selection of the 'best' side is also an element for painting one side only.
Actually after filing, préparing and cleaning, I spray the back side, and bottm with two thin layers of matte black acrylic painting; before starting the priming of te 'to-be-painted' face with diluted1:3 Humbrol diluted 1:3 (the first thin an very dluted layer, allows to immediately detect some final defects of filing, microholes,whcih escaped visual enklarged inspection of the metallic flat...)
Only decision to do = for dioramas, between two fighting communities, tray to systématically choose the same side for the given party, and fix the light source in the same direction (for me upfront left above) if exposed together, unless intentional.