Is resistor on LED's necessary after voltage drop?
as long as you don't exceed the forward voltage rating you wont need a resistor. it will limit the current going through itself, due to power rating. the problem comes from surges and over voltages. 

if you stack the LEDs for a 6.4 and so on voltage source what can happen is that one led will short out (melt internally and the magic stops) and then the rest of the LEDs are forced to endure the extra voltage across the junction and they all go bye bye.