On the back of the packaging of an LED from Radioshack
It goes to show you what muppets inhabit Radio Shack.
FW supply: 3.3V (typical), 4.0V (max.)
So that should read
FW voltage drop: 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)?
No you should use the average for calculating the resistor and then go check the current if it were to be the minimum and maximum.
Or is the best method for figuring out an LEDs voltage drop what Duane explained above?
Yes it works for that one diode but as they age this value will change.