The perfect expiry date, is one that is in the future - OK that's a trite answer, so let me qualify it. It takes about 5 days for a registrar transfer, add a couple of days for post auction chat, and I would say that you need 7 days from the receipt of payment. Most registrars will allow you to transfer an expired name, but you shouldn't rely on that. Some will sell your name and keep the money if it has expired.
Also you should consider that some buyers are influenced by the time to expiry, and if the name is desirable enough, it may be worth renewing it so that it has at least a year of life.