I have a 5k iMac, late 2015. It was delivered to me late May last year, so it's almost a year old.
It does happen, but only very little. After 20-40 minutes of browsing, no visible toolbar or anything, but if you change the desktop picture to a dark grey color, turn up the brightness and hide the dock, you can see a little burn-in of the icons in the dock. It disappears after a minute or two though.
I haven't had any problems seeing any ghosting on any of my desktop pictures, not even after a long period of work in things like iMovie, Adobe Premiere Pro or my screencasting application.
And if I didn't read the topics on MacRumors forums, I would never have done the Grey screen, hidden dock thing, to find out how bad it "ghosted", I really havn't noticed anything out of the ordinary.
I did have a 2013 MBPr 15" that did have a real burn-in problem, it happened after 2-3 minutes, and in a document it would leave a white box on my screen.
So I know what to look for, and spot it fast. But I only found out about my iMac, because I had to read a thread I found here on MacRumors forum
