I have never seen a FW supply listed in any data sheet for an LED, such a thing does not make any sense.
On the back of the packaging of an LED from Radioshack it lists a "FW supply: 3.3V (typical), 4.0V (max.)". Does this mean you should plan for and use resistors for this max setting (in this case, 4.0V)?
Or is the best method for figuring out an LEDs voltage drop what Duane explained above?