Clothing is definitely a matter of geographical presence. I once visited the Keams Canyon congregation in NE Arizona(on the Hopi indian reservation) for a week. It is the closest you can be in the USA to visiting a 3rd world country. For field service, the brothers and sisters wear denim pants, regular t-shirts, non-dress shoes, and no ties. Extremely casual -- picture yard work clothes. The indian people are so poor that they turn away the witnesses if they are dressed up at all. It makes them appear hoity-toity. The Mormons in the area dress up like they normally do and they pretty much get rejected. It was a very unique experience.
In my past halls, to be a book study reader or to say prayer, you had to wear a jacket of some sort. If I felt like skirting the duties, I would wear a cardigan sweater. I had a mixed wardrobe which consisted of both suits and sports jackets. I was never told about wearing only a suit to be allowed privs.
If you were handling mic's during the WT study, you had to have a WT in your hand while roving. That was kind of a pain. You couldn't leave it back at your seat. Embarrassing if you hadn't underlined a single sentence because you were partying the night before. It made it worse if you had a hangover and your meeting started in the morning. Your friends would smirk cause they knew what sort of condition you were in.
If you had a two-door car, you could NEVER drive in field service. This is funny because my used-to-be best friend bought a two-door Civic so that he wouldn't have to drive in service. He kept it until it went up in flames, literally, in his apartment parking lot, along with his motorcycle. Mind you, he was and still is an MS pursuing a degree in Elderism.