When I first started, we were required to change into our uniforms prior to starting work, and change back out of them prior to leaving.
Our contract stated that this would happen off of the clock, but it also stated that any employee who was required to change in and out of a uniform at work would be issued a locker. What happened was, we ran out of lockers so the rule became unenforceable and now most people wear their uniforms to and from work.
The other issue that came up was that, when shorts first became a uniform item in the early 90's, we had to buy them. Once a few guys bought them, they decided that they should be able to wear them to and from work since they were now personal property and the company had no right to dictate when and where we could wear them.