I don't leave feedback until I get the item and have inspected it. If I am the seller, I don't leave feedback until I hear back from the individual that they have recieved the item. The deal is not over until BOTH parties have gotten what they originally planned on. Therefore I hold the rating until then. That's just my opinion on this issue though. In the end, as long as the feedback is given (within a reasonable amount of time) I'm fine with it.
I feel the negative feedback is only there for those who intentionally screw you over. The neutral feedback should be left those who kept their end of the bargain, but not up to the standards that you expected. I've only left one negative feedback, and that was because the individual flaked on me and made me stand around a mall for 2 hours hoping he'd show. On the opposite end, I've had cases where I never recieved the package but still felt that the individual held up his end of the deal so I gave him a S+ even though I never got the package (damn Canada Post).