After reading What is negative voltage? and the answer given by kellenjb I wondered if I understood the topic sufficiently, and started theorizing. If the + or – sign is just a convention and the voltage is the potential difference to ground, and my PSU has a +12V, -12V and GND connector, would it make no difference wether I connected a circuit to +12V and GND or to GND and -12V? In other words, say I had a 12V LED, could I connect the anode to GND and the cathode to -12V and still have it work same as when I connect the anode to +12V and the cathode to GND?
Electrical – Is connecting a 12V circuit to +12V and GND the same as connecting it to GND and -12V
groundvoltage
Related Solutions
You can do it either way. Each LED can be its own loop, or you can do it in 
In series-parallel, you have multiple current loops. Each loop can carry as many LEDs as you can power from your supply voltage. For example, if you had LEDs with a nominal 1.7Vf, then you would have 5 LEDs for a total of 8.5V. With a power supply voltage of 12V, a reasonable resistor choice for each branch would be (12 - 8.5)/.02 = 190. Closest standard 5% resistor value I can think of is 220 ohms which would give you 16mA per branch. Should be plenty bright enough.
Voltage is a difference of electric potential. To simplify, it is an measure of the difference in concentration of electrons between two points.
Volts is a unit of measure of voltage.
It would be pretty difficult to have exactly no electron on an electrode, and as the voltage is the difference of potential, it is always relative to somthing. Usually, we say that a given electrode has \$X\$ volts and that implied relative to the ground of the circuit. The voltage of the ground is 0 volts, because it is referenced to itself. However, if you measure the voltage of a ground in one circuit, as a reference of another ground in another circuit, you may see some difference (non-zero).
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Best Answer
Yes, the LED would work.
Think of it like this: Voltage is basically the difference between the potential of two nodes.
So if you have the anode to +12V and the cathode to GND (which is basically our 0V reference point) then the LED will see a voltage of +12V-0V=+12V
But then, if you have the anode to GND and the cathode to -12V the LED will see a voltage of 0V-(-12V)=+12V. As you see, there is no difference in practice.
Also you say:
That is true for an absolute value of voltage, like the +12 or -12V of your PSU; they are referenced to its GND terminal. But as I said before, voltage is actually the potential difference between two points. In this sense, the absolute voltage of the anode and cathode of the LED is not important.