We adopted a policy of giving feedback to buyers 30 days after their purchase.  If there turned out to be an issue with a product or a pain-in-**bleep** buyer, it provided some breathing space.  We always give feedback, but 3 out of 4 buyers never bother.

On the Chinese eCommerce sites buyers & sellers receive automatic positive feedback 30 days after sale if either party has not done so and eBay should consider doing same.