I’m gonna say as long as your hygiene’s good and whatever your wearing has a pocket for your wallet you’ll be ok.

Casual is cool

Can never go wrong with clean casual clothess.Jeans & shirt  always in style.Blending in  and not standing out is also advisable,

I agree with the other response about the attire depending on the situation, location, occasion, time of day, etc... Since being discreet and not drawing attention is usually the goal, then it follows that dressing appropriately according to where you're headed & trying to wear something that "blends in" or dressing casually is usually a pretty safe bet.  

For me, I'm way more concerned with "hygiene" than I am with attire.  If my client shows up with his hygiene on point -- freshly showered, well-groomed, smelling nice, etc, then I could honestly care less about what he's wearing since it will be coming off pretty quickly anyway I think it's safe to say that showing up in casual attire with your hygiene in check is the way to go and will make most providers very happy to see you.