Can I keep the MacBook Pro Retina's power-cord plugged in at all time?
Well, Apple does not recommend leaving your portable plugged in all the time, as for a lithium-based battery, it's important to keep the electrons in it moving occasionally. MacBook / Pro, though, are designed to deliver up to 1000 full charge and discharge cycles before it reaches 80% of its original capacity. 

A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could listen to your iPod for a few hours one day, using half its power, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so you may take several days to complete a cycle. Each time you complete a charge cycle, it diminishes battery capacity slightly.

Since 2009-2010 laptops carry a technology called Adaptive Charging which basically is a chip within the battery that talks to its cells monitoring everything from charge level to temperature. The chip relays this information to the computer, which uses an advanced algorithm to determine the optimal charging current. Adaptive Charging reduces the wear and tear on the battery giving it a lifespan of up to 5 years.

So, I'll actually won't worry that much about the battery. Just try not to keep it plugged all the time, and remember to do a full charge cycle at least once per month.