max forward current is usually about 10 - 25 mA, which occures with about 1.5V across the junction, so for 5V supply that requires you to drop 3.5V @ 25mA.
Using Ohm's law where V=IR, gives V/I=R so;
3.5/0.025= 140 Ohms
and if it's wrong feel free to shoot me, havent bothered checking it and this is the first time in over 12 years I've used it
Real reason is that a reverse voltage of much more than 25V will blow most LED's (VRmax
). BUT as long as the voltage is less than this (and it should be!) then they'll be fine.