The forward voltage of LEDs depends on the temperature, the current flowing, the manufacturing process and so forth. Expect the actual forward voltage to be +/- 15% of the nominal voltage from the datasheet...
And drive LEDs with (limited) current, not voltage because the temperature dependence can lead to thermal runaway otherwise (the forward voltage for a given current falls as the chip heats up (so the current for constant voltage increases).
The figure you really want from the datasheet is the minimum forward voltage for the temperature range you are interested in - you might find a graph of forward voltage against temp if lucky, but its probably average not worst-case...
The other approach is S.o.T. (select on test) - try different resistors to set the current for the actual device as you assemble the circuit...