I was thinking of general 6v operation- 20-30 mA is fine those small LEDs. If less resistance is needed to fix it then you'll have some brighter LEDs is all. Probably shorter lifespan, but oh well...
But don't forget about 5V will be dropped by the LEDs, leaving about 1.1V to drop across the resistor. 100 ohms would limit current to about 11ma.
Patrick del Castillo
President, Principle Engineer
Castle Creations