You should use some resistors. The math for calculating what size resistors is pretty easy. Also pages like this,http://led.linear1.org/1led.wizhttp://led.linear1.org/led.wizcan do the math for you. If you know how much current you want to flow through the LEDs, then use that number. Otherwise, I suggest 20 milliamperes (20 mA) as a typical, safe but still bright, amount of current for a 5 mm size LED.