I'm not a professional detailer, but I am a potential customer, (at least I was before I found this site) so I'll offer up my perspective:
1. If you're doing customer contact, a logo polo and no jeans is a minimum. If you're picking up a customer's car from a downtown location, let local business custom be your guide. When I worked in DC, you better not come into an attorney's office for his Jag without at least a blazer on. Here in Hawaii, an Aloha shirt and khakis are fine. Detailing is an image buisness, right? Much more so than lawn care, carpet cleaning, home repair, etc. Even with home repair, the guys who are really cleaning up are the franchised guys with a brand name people can trust, and they all wear uniforms. Think Sears, Home Depot, Lowes, mobile crews.
2. Logo T for anybody doing work in public, (mobile detailing, or parking lot detailing)where your employees are seen by the public and customer. If you have a shop that is mostly hidden from view, then nobody cares.
3. No jeans on anybody. Buttons and rivits scratch, a lot of your customers are going to know that.
4. Don't overdo it. Keep the logos tasteful, simple and uncluttered. Also, don't load up on the logo'd items. A shirt and visor are enough, I'm not looking for a NASCAR driver to come detail my car. And no ball caps. Ugh, ball caps.
5. Keep uniforms neat and in good repair. I will notice that torn greasy shirt. Mended would be okay, but stained, ripped and wrinkled is a no go.
6. Employees should be neat. Facial hair trimmed/shaved, long hair pulled back. Don't care about your appearance, won't care about my car's appearance.
7. Duh, make sure your own car or work-van is clean!