As long as you're driving the series string of LEDs with a Current Regulator [i.e. Constant Current], then the only consideration for the LED Forward Voltage is, whether the sum of the Forward Voltages [the highest Forward Voltage they will ever be -- because it changes with temperature], of the LEDs, is less than the maximum voltage the Current Regulator can supply and still be within its ability to regulate the current. Other than that, the Forward Voltage on an LED has no bearing on how an LED is properly driven.
AND, as long as the optimal current is the same for each of the LEDs in series.