Hygiene is one thing and that's gross - but the rest of the post I have to disagree with.
Casual Fridays is just that. Casual. I work for an employer whereby we are asked to be ourselves in terms of dress at work, as long as it isn't dirty or offensive we can wear what we like - including hats, sandals, hoodies, jeans and trainers. They are casual for a reason.
I couldn't think of anything worse than having to consistently wear business dress to appease people when I can dress and be myself both in and outside of work. Just because you're wearing jeans and trainers, it doesn't have to look unkempt, it's easy to look smart casual in that sort of thing.