Background: I built a 10x20 LED matrix for use inside a jack-o-lantern. The matrix is driven by 74HC595 shift registers that are connected to an Arduino.
I've attached the Vcc of the shift registers to the 3.3V port of the arduino, so the output pins of the shift registers will be 3.3V when set high.
The datasheet for the shift register says each output pin can source at most 20ma, which is exactly the nominal current of my 3.3mm orange leds.
I know that power supplies can source much more than 20ma, so the use of current limiting resistors are necessary when the voltage is higher than the forward voltage, however is there any harm in exceeding the forward voltage if the current is clamped to the LED's nominal current rating?