Off; because, why?
iMessage tells you if it was delivered or not, so why do I need to know if the person read it or not? They'll get it and respond when they feel like it.
That's how I feel. Also, Read Receipts are not an accurate depiction anyways as people can read messages via Notification Center and it won't mark the message as 'Read'.
I've had them on since iMessage was introduced... but after months of having it on I realized that there are people that I want to ignore for as long as i can lol... so on or off?