I have to say, that I work for the Army as a Civillian. I was hired on after being a contractor for some time, where there was no dress code for contractors.
Even though I am doing essentially the same job, now that I am a civillian, there is a Dress Code. I wear a Tie, Slacks and a dress shirt every single day. I work around Computers, but it's about direction from the top down. I made the mistake of not wearing a tie on my first Friday there. I had a Sweater over my button down shirt and slacks.
It didn't matter. I got sent home like a little kid from school to go and grab a tie. I never made the same mistake again!