here's the info I found on spark arrestor resistors
http://scriptasylum.com/rc_speed/nospark.html
http://rc.runryder.com/helicopter/t547501p1/
They seem to suggest keeping it hooked up and say that electricity like water will take the route of least resistance and therefore the resistor hooked up doesn't matter.
I haven't messed with a resistor yet...will be a while as my XL2 crapped out