Or are the stories overblown?
Stories are not overblown.
You shouldn't use any MBP (except for the classic non-unibody ones) in clamshell with either Caffeine or No Sleep and try to run processor-intensive applications. It's because the computer has a MUCH HARDER time trying to vent hot air when the display is closed.
To be more specific, air vents are directly where the screen hinge is. Once you close the hinge, you block the vents, and consequently, the ability of the computer to dissipate heat. This has been a problem since the introduction of the unibody design.
The all aluminum body can still dissipate some heat, but if people try to insulate it by putting on full-body skins or leather or stickers, etc... then the ability of the aluminum body to transmit heat is significantly hampered.
I have used many unibody MacBooks and came to this conclusion. Basically, if you can't stand leaving your expensive gear naked without any full-body protection, then absolutely avoid operating it in clamshell. If you absolutely must operate in clamshell, do try to limit your activity, and do try to leave the body as naked as possible.
If the rMBP is used like any laptop should be used (screen propped up at a comfortable angle with your line of sight), then there shouldn't be any concern about heat.
In fact, I'd say the rMBP has the best cooling system I have seen on a portable device... ever... when it's operated like a normal laptop, that is.