Feedback should be left when item is recieved as described. If it is not received as described than the buyer should contact the seller to explain what he/she feels is the discrepency (or the wrong doing).
Most often settlements can be made without the issue escalating. If the results of a voiced complaint are unresolved negative feedback should be left. I believe the system is unfair in the way the seller is allowed to "even the score" by responding with negative feedback to the same buyer who has promptly paid, received an item in less than satisfactory condition, and tried to settle the matter fairly and up front. After a period of time you cannot see whether negative feedback was left by "buyer" or "seller."
If I've asked a question before bidding, or have asked the seller to give me specific details about grading beforehand, and have been decieved in any way, I'll leave neutral feedback without prior warning. I believe there are many problems with the system as it stands (including not being able to report fraudulent sales activity such as manufactured miscut errors from sheets).