I have seen some military personnel wearing watches upside down, however, majority of military personnel wears electronic watches on top of the wrist and have it covered with sleeves.
WW1 trench watches had a utility - time of the attacks were timed and they had no radios. With the invent of portable radios and communication equipment, utility of wrist watch to start military operations significantly diminished.
With that said, WW1 trench watches were just pocket watches on the wide wrist band. They aren't comfortable to wear in all positions, so I highly doubt that even during WW1, a lot of soldiers would wear their watches upside down.
WW1 trench watches had a utility - time of the attacks were timed and they had no radios. With the invent of portable radios and communication equipment, utility of wrist watch to start military operations significantly diminished.
With that said, WW1 trench watches were just pocket watches on the wide wrist band. They aren't comfortable to wear in all positions, so I highly doubt that even during WW1, a lot of soldiers would wear their watches upside down.