I think it really depends on the uniforms. At my daughters' elementary school we are having volunterary uniform year...the parents will vote on it again I think next year to see if we will make it mandatory.
Today was the first day of school. When I picked up the girls, about 70% of the kids came out of the school wearing selections from the dress code...but it isn't one uniform, it is tan or blue pants or skorts or shorts, white button down or polo shirt, or red or blue polo shirts, or there is a certain plaid dress or skirt that the girls can wear.
I guess this is more of a limited dress code than a uniform. We haven't bought new school clothes, because I wanted them to see what the other kids were wearing. they can wear their summer clothes for the first month of school any way hahah. My 7 year old wants all her new clothes to be the uniform clothes, and my 9 year old said no way. The 5 year old didn't care hahah.
Our school has well to do and poor kids in it, and you couldn't tell by the clothes who were what. I do wish that we had a dress code or uniforms like this when I was a kid because I was constantly berated for my cheap clothes. The clothes we can buy from the list is much cheaper than most of the other clothes.
I think a strict dress code or liberal uniform can make a difference for the poor kids feeling a little better about their clothes.
However, the schools that go with very limited uniforms may be harder on poor families if you are limited as to where you can buy the clothes.