I used to assign each piece individually as I usually run into the situation where my soldiers run around partly equipped.
Main reason for this in my opinion is that when I assign a uniform and have more than 2 squads with the same setup (e.x. Steel armor, helm, greaves, boots, gauntlets) and I produce a new batch of armor everyone who likes to "upgrade" drops his old uniform piece and runs to the stockpile.
In most cases this is successful but sometimes I guess there is a mismatch and two or more soldiers run for the new helm. That shouldn't be a problem but they refuse to pick up their old equipment when they recognize the fact their new helm is already in possion of someone else.
Most problematic is when it goes to boots and gauntlets *sigh*.
Running around with only one is common within my stubborn minions
Recently I made the experience that the tedious assignment work was to my advantage when it comes to mass equipment drops from squads returning from raids.
While it usually ended in a complete mess until everyone got a piece of equipment its now more smooth.
This was in the past even more messy when not only the returning squad tried to get back its equipment but also everyone else with a uniform matching the upgrade criteria.
At the moment I am quite content even with the initial invest per squad.
I am just looking for a chance to mod a hood and cloak just for the military. As it is counted as clothes it has wear and new ones are picked up and stored by anyone who likes to aquire new clothes. I guess that is part of the
''Need'' system.
At the Moment I am running six full squads with all equipment personal assigned and I am going to draft my civilians too (mostly for safety reasons but for their needs to train too)
At the end I think I gained most of control over my clothes industry and the whole equipment problems.
Maybe a tip for making personal assignment less painful is to forbid all clothes an armor which is already in inventory or owned (via enhanced stockpile view)
That leaves only your newest/unused equipment in the list (beside the bug with the corrupted items from raids)
All other metal from Siegers is usually marked for melting so I get my uniform assigned pretty fast and without problems.