At every job I've ever had (other than one where I had a strict uniform no matter what day of the week it was) we've observed "casual Friday". I don't know what it is about the last workday of the week that makes certain items of clothing okay that are generally frowned upon on the other four working days of the week. I'm not complaining at all, it is nice to have the ease of knowing I can pull out a pair of jeans every Friday. That's about 50% of my clothes planned way ahead of time. :D
I am the only female in my office so my dress code is pretty much up to me. Personally, I think when you work with any other human being it's polite to still put in some effort, even on more casual days. Jeans don't necessarily mean sloppy. I've posted before that I mostly have dressy clothing since I wear them six of the seven days of the week. So for "casual Friday" I wear one of my "normal" top ensembles with some well fitting, figure flattering denim and call it good.
A lot of people assume I work in the fashion industry because of how I dress, but I actually work in an insurance office. I think it's important no matter where you work or what you do to show a little respect for yourself and those around you who have to look at you.
It doesn't take a ton of money or even a lot of time to look nice. The more pride you have in yourself, the more pride you'll have in your job and even your life.