Is it okay to leave my MBPr 15" always plugged in to external displays?

I don't think your monitors and peripherals have anything to do with this heat problem. Make sure you go to Apple (symbol at top left) > sleep when you are not using the laptop. This prevents from any background programs from heavily using the CPU or other components. If that doesn't work, open activity monitor and click on the "CPU" tab and check if a program is using a lot of your processing program. If that program is a known program to you and you know it is supposed to be using resources more heavily, leave it running. If the program is unknown to you and you feel that it could be a background application that is unwanted, quit the process. Try to avoid quitting system programs; those are needed and can sometimes, but rarely, use up quite some CPU. But that is usually only when you instructed it to do so; for example copying gigabytes of files to a disk.

Lots of info here, but hope this helps! Ask any further questions that you may have!

-24GHz

If you keep the lid closed this reduces some heat loss thus causing the computer to operate hotter. Operating in clamshell mode 100% of the time is not recommended practice. It is never good practice to leave a computer turned on or asleep for long periods of time. They should be shutdown, especially when left unattended.

The computer will shut itself down if it starts getting too hot. They're smart like that. Nothing to worry about.

Not so. Operating at high temperatures waiting for the computer to reach thermal shutdown imposes a lot of heat on the motherboard and other components thus reducing their useful lives. Never take "getting too hot" casually. It's a sign of a problem or simply misusing the computer.