Neville you got it on target. Some of our boys serving as bochurim shluchim in yeshivos and thus are role models for the bochurim will prefer the not tucked in shirt to convey that bochurim should not obsess with their appearance. The vast majority of the men do tuck in as it looks more mesudar. I guess they tuck.in for dates and after marriage. Of course there are bochurim who come from home where being mesudar in all areas is stressed so they will tuck in
and there are married men who don’t, mainly fiery Israelis. It’s not an obsession either way but there is a message there…
I guess another way this tone was set was the Rebbe wearing a tie but strongly encouraging his chassidim not to. Same idea behind.