What if I get a single red LED that is rated for 5v and I power it from a 5v power supply? Is a current limiting resistor needed then?
Now that is an interesting question. Red LEDs typically have a (forward) voltage drop of around half that or less. If you actually had one rated at 5V, I would suspect there is already some mechanism in place to safeguard the LED ... perhaps a resistor, or even a basic buck converter.
If you were to change your question and say:
What if I get a single red LED that is rated for 1.8v and I power it from a 1.8v power supply? Is a current limiting resistor needed then? Then the answer is: It depends.
I'm trying to understand conceptually when/why you need a "current limiting resistor".
This is a common question that many have asked - and it is a very fair question.
I would point you to the chart above and get you to pick one colour - say the red - and look at the curve very closely. What you are looking for is the change in current as you increase the voltage. Start at 0.5V and see what happens to the current as you go up in, say, quarter volt increments. When you get to 1.75V, you can see a trend beginning ... a trend that goes right off the chart (quite literally!) in another two steps.
While not perfect, I do like to use the water analogy - and my offering for this is as follows...
A resistor is like a pipe with water flowing through it. A bigger pipe has less resistance to water flow - so a bigger current of water can flow for a given pressure, than will flow in a smaller pipe.
An LED is more like a dam (with a slightly uneven top and some other little idiosyncrasies). As the water level rises, water starts flowing over the lower points. As the water level rises a little bit more, the water flows over more of the dam wall and the volume increases noticeably. Once it gets to the point that water is flowing over all the dam wall, then even a tiny increase in water height will result in a massively increased flow of water. There is, effectively, nothing to limit the volume of water that flows over the top of the dam.
Looking back, you will see that the volume of water flowing changes very dramatically over a relatively small range of water height. If you chart this, you can see a definite area where this happens - and it will look somewhat similar to the LED chart above.
Back to the LED. While there is this "dam" characteristic, there is a point where the current through the LED is just going to destroy it, so the challenge is to limit the current that could possibly flow. Adding a resistor does exactly that.