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
There are two kinds of threads that I either ignore completely or take them with a grain of salt. The first one is burn in (ghosting) which happens to practically every monitor (or TV) that has ever been made. I remember playing video games on the TV where some instruction manuals would even say don't leave the game stand still for too long. The second is dead pixels that is the topic that irks me the most, for most of the time (not all) it's someone finding one single dead pixel, doing a screen capture (or even taking a picture of it) and posting it with a circle around it. This one dead pixel is just one tiny pixel out of millions of pixels that the monitor displays and yet the person is having a hissing fit over it. It's no wonder that computer companies put a minimum of dead pixels allowed before they will do anything about it.
lol Pepster, it's great you've got such low standards. You can use all the refurb and sub par components since you're cool with everything should take the words of wisdom from your signature since you mention you avoid these threads.
To answer the OP I've seen varying degrees of screen burn in/image retention which means there are definitely variances in panel quality. That being said, there are panels out there that do not exhibit any; in this day and age receiving a panel with dead pixels and noticeable degrees of rentention is unacceptable. LCD tech is now new, and is the sign of a cheap or inconsistently produced panel.
Ok thanks everyone. It seemed like in 2012 when I bought my imac there was a very large amount of threads about the ghosting. However since the 5k imacs it doesn't seem like as many complained about it or maybe everyone is used to it now.