Picture is attached, I found it on the link below (along with the discussion)
www.airliners.net/aviation-forums/tech_ops/read.main/199172/
I was having this dilemma after avoiding them for so long, but it would appear to be - whatever the custom is at the training school you are at. The Airline schools tend to use them.
So it would appear pre flight test (even PPL is a sign of respect to the instructor (yes the US really goes out of their way to show respect for the checkride FAA designee), the best thing you could do is find out if they 1) use stripes and then like wearing a tie at a business function, 2) wear the least amount appropriate to your rating (so for me that would be one silver stripe). Does that make clothes an option?
I wish I could be that funny (see pic)
OK Back to my studies but you really gotta see the pic - what did he say, if you're gonna do it, do it all the way...(I am joking now, really...) PS: Bob the coffee machine was off, (no one has posted for three months, better service the machine) maybe you need an alternate forum called "Around the Bar"
for example if someone I knew personally were doing their biennial renewal and they were purely recreational pilots, I couldn't care less if they were dressed informally. But if it was their initially PPL test (and they were intending to go CPL), it made a good impression if they dressed the part. 