I'm posting this just to make things clear.
A led is designed to just let power through in ONE direction.
So if you put the led in the wrong way, it wont short the system.
It might damage your led though, depending on how great the voltage is.
Most leds are rated with a backpower limit(don't know if this is the right term
For example, a blue led I have is rated for 3.3V/(3.7V MAX), 20mA, and 5,0V backpower.
But this isn't really a problem since i you only connect the led for a short time, and then see its not powered up and change the polarity. It wont damage your led or system.