Never, like prodding a sleeping lion, when woken he will probably bite you, leave him alone.
Prodded buyers can leave poor feedback (e.g. your were a pest) or in the now dangerous days of DSR glowing positive feedback and all 1 star ratings that dent you without you being able to defend yourself. Then eBay will savage you in searches or worse.
Another problem you can have is customers keep on coming back for more, my feedback profile is almost static due to loyal customers.
But then again, the best rating you can have is buyers coming back again and again, because if they return they were obviously well pleased with you, not just saying that to ensure apositive by return. An intelligent prospective buyer will see this.
P.S. Yes, aways best : buyer feedback then seller feedback.