LEDs are rated for current. Their forward voltage is a wide range of voltages so you must measure the forward voltage to calculate a current-limiting resistor for it.
Assuming the lower forward voltage will result in a conservative (higher) estimate for the resistor. However, if your supply voltage is not close to 3V (like >6 V), using either value is not going to make much of a difference.
That said, it wouldn't hurt to actually measure it.
Okay then I'll measure it, is a constant current source required to measure the forward voltage drop or can I just use a safety resistor which is in a reasonable range ? It's a little embarrassing to have to ask such questions about an LED, but I never really looked into them that much.
Use a proper current source rather than a simple series resistor, then you won't have to worry about the forward voltage providing your regulator can provide the voltage at the lowest supply voltage.
True, I was thinking of just testing how bright it was untill I can get a proper regulator setup, but you're right I may aswell just get a regulator sorted out now - It would be a different brightness anyway.