I usually (99 times out of 100) leave feedback first - on the day I have posted a buyers parcel. 

The opportunity to leave feedback, provides a further opportunity for me to say thank-you to the buyer, rate their payment speed and confirm (and put on the public record) actual post date of item.

For multiple purchases I usually leave feedback for all but one of the items. More of habit than having a clear rationale to withhold just one.

From time to time if I strike a buyer that has been a very, very late payer, has been very rude or aggressive or way too liberal in dishing out negatives to other seemingly great sellers, I do not leave feedback until they leave it for me. 

I have confidence in the items I sell and that my descriptions are accurate, I am also confident with my ability to work with a buyer to resolve any issues that may arise, whether that be post, misunderstandings or dissatisfaction with their purchase. 

I also have a level of confidence of ebay in this particular area who have removed  a couple of negatives from this particular ID when it was clear the buyers had not read the listing descriptions.