As a seller, I leave it once the sale is completed. I consider it completed once the buyer has their item and all is well. I use vrane.com to leave my feedback, and they only leave to positives. If someone emails me asking about feedback, I tell them the above. If I didn't use vrane I would leave it once I heard from the buyer that all was well, and not just once the feedback was left.
From eBays tutorial. You can pick and choose what is best for you..here is the gist:
http://pages.ebay.com/help/tutorial/feedbacktutorial/intro.html:
Each member may choose the best approach to leaving
feedback for them. There are different philosophies on
whether the buyer or seller should leave feedback first.
If the approach your buyer or seller takes matters to you,
review the comments left for others in their Member Profile
page to see when they leave feedback. Each member must
decide how best to manage their reputation. eBay's internal
research has shown that sellers who leave feedback upon
payment receive a higher percentage of feedback compared
to those that wait for the buyer to leave feedback first as
confirmation that they are satisfied, but either approach is
acceptable. One reason that sellers leave feedback upon
receipt of payment is that they believe that the buyer has
upheld his or her end of the transaction. One reason sellers
wait until a buyer has left feedback is they take the feedback
as the signal that the buyer is satisfied with the purchase and
no further customer support is needed. Other sellers use
automated tools to leave feedback and wait until feedback
has been left to leave a comment. Regardless of who leaves
feedback first, eBay encourages both members to always
leave appropriate feedback.