I almost never buy extended warranties, but I think the high cost of the iMac and the all-in-one form factor may make AppleCare worthwhile.
I was curious what Apple charges for out-of-warranty Mac repairs and could only find the paragraph quoted below. I don't know what it costs to replace the screen in a 27-inch iMac but I bet it's a bunch.
"How much will it cost? There's no charge if the issue is covered under warranty, AppleCare+, or consumer law. If your issue isn’t covered, the price depends on the type of repair. Ask your service representative for an estimate."