Once you've balanced one prop you will soon balance all your props. The reason for this is that you'll see the difference immediately and if you have a GPS you can even measure it and see that it actually works. How much you will gain by balancing your prop depends on what kind of prop, manufacturer and a bunch of other things but usually you will get 5-10 km/h extra when you're done with it, if you had a new, untouched prop before.
But that's not the only good that comes from balancing. If your blades have the same weight, size and sharpening it will give your motor a more consistent load which reduce losses in the motor which also increases your motors life.