Please note that according to the very active thread that I link to below, as recently as last week some consumers are still being turned away with the 'cosmetic damage' line and told to foot the bill themselves, while others are seeing repairs done outside of warranty. Since there is not yet any official stance on this from Apple, at this stage you are by no means guaranteed to be covered by Apple Care should your model develop signs of this defect.
As far as the 2015 model goes, it's still too early to say, in most cases that have been reported so far, the problem seems only to have manifest itself after about 8-12 months or more of regular use, so if nothing in the manufacturing process has changed then reports should start surfacing soon enough.
From reading through this thread on the support forums (Re: My Retina Display has stain damage, HELP!!!) there are more and more people with the 2014 model who's coatings are also now starting to fail. If you read through some of the more recent postings you will also find a link to a German magazine which claims that the 2015 model appears not to have the coating applied at all, which I can neither confirm nor deny, but certainly had me raising an eyebrow.
My own late 2013 model only started to peel off after 18 months (yesterday, hence my late reply to your post), but then I've seldom had any reason to clean it since it's mostly used in clamshell mode on my desk and I've babied it from the day I purchased it, including cleaning it as per Apple's recommendations using the supplied, apple-branded cloth, sometimes slightly moistened with a few drops of water.
Best of luck in your buying decision, I'd be very wary of any retina macbook purchase before hearing something official from Apple on this topic.
