Another thing about series LEDs. The point about the forward voltage drop is spot on, but after adding them all together you have to leave a bit of voltage headroom for the resistor to be able to do its job. The LEDs act like Zener diodes in that they each drop a specific voltage over a wide range of current, but the battery voltage is not constant. If there is a very low voltage difference, the voltage across the resistor can vary a lot, this will cause the current through the circuit to vary, and thus the brightness.
There are not many ways to damage LEDs - the top one is too much current (i.e. not enough resistance for the voltage) then too much reverse voltage (they don't like it up 'em, to quote Corporal Jones).