I belong to a casting club, so several members are quite experienced, and as well we have some new casters join each year. I agree with the first response that the rod and reel system should roughly balance on your index finger when in the normal casting position. As well, I agree the rod should be balanced for the time you are drifting/mending/working the fly. This is actually most of the fishing time - casting is not most of the time. If the rod is not properly balanced the majority of the time you will be using muscles either lifting the tip up (out of the wate or higher) or pushing it down to keep the tip near the water. Minute by minute it doesn't seem like much, but after a couple of hours ... tiring.
However I disagree that with today's lightweight rods balance is not much of an issue. Too often I see outfits with a modern rod and a reel that is way too light. A tip heavy rod can't always be accommodated by moving your hand to the top of the cork. And casting a tip heavy rod is like trying to cast a baseball bat - really hard on the elbow.
If your reel is too light, a wrap or two of heavy solder around the arbor of the reel under the backing will create the balance you need, and also keep the reel from rotating out of balance. The rod and reel system will have more mass, but will actually feel lighter in use - because it is balanced. My $0.02.