I can't give any good info on whether or not such issues have been solved by Apple/LG or not, but I would definitely advise you to go ahead and run a few subjective tests to make sure you're happy with it before your return window closes.
Here's a simple way to test for image retention issues:
http://www.marco.org/rmbp-irtest.html
As far as the LG screens being discolored or whatever, I guess you gotta determine if it looks good to your eye for yourself. If it doesn't look bad to you, don't worry about it.
That said, I always check any new device with a screen by using some solid color slides... I like to use Keynote to create slides the exact resolution of the screen in question, but however you can fill your entire screen with a uniform image of one color works. I like to hit all the basics: white, grey(s), black, red, green, blue, yellow, cyan, magenta, and brown(s).
Viewing the entire screen full of one solid color will make it easier for your eyes to pick up on any flaws that will ruin your experience later. Seeing all the different colors will help ensure you don't run into a "everything is fine until there is a lot of purple on the screen" (or whatever) type issues later on. Greys, in particular, are where backlight uniformity issues tend to become most obvious most of the time, at least to my eyes, so be sure to at least view a few shades (25%, 50%, and 75%).
Then look at a variety of other content. Look at a few more still images, but "complex" ones this time. If you wanna look at calibration images, go for it. Viewing some images that contain standard gradients wouldn't hurt. I prefer to just check out some pictures with lots of varied content; landscapes, portraits, still life studies, whatever. Go exploring on DeviantArt for a while; that site is great for viewing all kinds of different stuff, in addition to being quite enjoyable to wander around in for a while from time to time.
Lastly, just use your MacBook as you normally would. Watch some videos, just to make sure you don't notice any problems there; surf the forums here and a few other places (scrolling through text will often reveal screen issues when you're not looking for them); play some games if that's your thing.
Basically, aside from a few fairly basic evaluations you should do with any new product, just spend time with your new baby to make sure it's healthy. ;D
Congrats!
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Haha
Well said. You posted the TL;DR version of my post in the time it took me to get the way-too-wordy version typed out.