I normally just ignore that the LED is in the circuit and select the series resistor to limit the current to the required value when placed across the supply voltage.
Ignoring the LED voltage drop doesn't much matter when the supply voltage is much higher than the LED drop. But if the supply voltage is 2V you definately need to consider the LED voltage drop. For a red LED (1.4V) you'd need a 40 ohm resistor to pass 15ma thru the LED. Ignore the LED drop and select a resistor based on 2V/15ma = 133 ohms, and use it with the red LED and you'll only get 4.5ma thru the LED. The LED will be much dimmer than desired.
