This isn't asking the equation to determine the necessary resistor for LEDs, but more asking the general practice for selecting them.
I've seen multiple circuits that have used much higher resistor values than what I would deem to be necessary. For example, I have seen a design that has used a \$330\Omega\$ resistor for a red LED with a forward voltage of \$2V\$, and forward current of \$20mA\$ on a circuit with \$5V\$ supply. By my calculations that's twice as high as it needs to be (\$150\Omega\$).
I've read else where that this resistor is the 'playing it safe' choice, in that they use that wherever and can be confident they wont blow the LED. But is there any other reason behind it? Other than purposely halving the LED brightness.
Perhaps this prolongs the life of the LED? In my circuit I've selected the theoretical correct resistor value for each LED, but want to know if there's a practical rule I'm missing as the resistor values are quite small at times.