I definitely don't think someone ought to ask for feedback after doing a trade, even if they left one for you first.  That's happened to me before, and at the very least it's annoying and at worst it raises red flags (as it should if you're on DT). 

If someone asks for positive feedback after doing a very small trade or if they had nothing at risk, you probably should be suspicious that they're trying to farm trust.  It's happened many times over the years, and I'm sure people are still trying to do it.