Regarding, "It never hurts to ask" for positive feedback. I disagree. When I have something to say, I leave feedback. When I don't have anything to say, I don't.
It may be against the "community values" you're speaking of, but sometimes I just don't have any feedback. Maybe I ordered something for my mom and didn't hear back. Maybe the service wasn't stellar, but was also not worth complaining about.
Either way, I have had two sellers pester me to the point of being really and truly annoying (7-8 requests). You know, if a buyer doesn't respond after two polite requests, there may be a reason. You should forget it. One positive feedback is not worth risking pushing a buyer to anoint you with a negative feedback instead.
The two annoying sellers got lucky. Bouncing their emails was as mad as I got.
Just a thought.