I have a 2012 15" rMBP purchased in November 2012. It had the LG screen and developed bad IR after about six months. Apple replaced the screen with a Samsung and I've had no IR at all over the 2+ years I've had it since. I haven't heard too much about it (but haven't been looking for it either) on the newer machines so maybe it's not a big issue anymore.
May I ask why you're thinking about replacing your 2012? When the new 2015 models came out I briefly thought about it as well, but then realized my 2012 is running so smoothly and continues to meet all my needs. I think I might upgrade in a year or two, perhaps if they do a redesign. But the 2012 has been, and continues to be, a great machine for me.