It is very important. At the RPM's that typical model engines and motors (gas/fuel/electric) turn these props, it can vibrate to the point that can go as far as to destroy a model in a short period of time. Lesser vibrations can and do damage receivers, shake connections loose, break down batteries.. in short order too!

It only takes a few moments to balance a prop, and it is well worth it. Most modern props are pretty much in balance, and you will find that when you buy them, but occasionally you will run into one that is off balance.

Also, consider a landing that was less than perfect. When you 'hit the ground', the prop hits the ground too and it stalls the motor and, worse yet, chips the tip of one side of a two bladed prop. Ok.. now what happens? Well, that chunk of prop is now missing causing one side to be slightly heavier than the other... off balance... vibration time!! So, take it off, put it on the balance gig, then trim off the heavier side to balance it.... or replace it.

DS.