A nice kind of rule I go by: If the way I am playing or the way my opponent is playing is by trying to end the game quickly, often I won't the see the game long enough to get the benefits of an extra channeling as opposed to just saving my mana, or casting things that will help my economy without increasing my channeling such as ring of beasts, arcane ring, etc. If I am playing for the long game, I will usually try to increase my channeling to the average amount of mana I will need a turn. I do this by actually planning out my openings, adding up the two cards I cast per round (If I do cast two cards), and then divide by the number of rounds I added. Thus:
Round 1: Mana Crystal+Mana Crystal= 10
Round 2: Grizzly+ Ring of beasts= 18
Round 3: Hurl boulder+ Hurl Rock= 12
Sum of all the parts: 40/3= 13.333 channeling.
This number is a good number for me to use if when I want to start banking my mana. I know that I will probably use around 13-14 mana a turn, so, if I am channeling 12, I will want to probably save 2-3 mana a round at least. This obviously becomes skewed based on the opponent and what he is doing, but as a base number, it has really helped me to figure out the mana I am using, and how to best conserve it. As far as how much channeling to gain, I usually want to gain enough channeling to where I only have to save 1-2 mana a turn in order to meet the average mana consumption a round.
What we need to keep in mind as players, though, is that no matter how efficiently we spend our mana, what isn't being used to better your position to kill the mage is wasted mana and time. the more time we spend building a good channeling is the more time we spend not killing the mage. Now, there are some cases where building a really high channeling is the key to winning (see Mana Denial Wizard), but this is not the norm, and should be taken into account as such.
There are 4 different kinds of resources in the game: Damage, mana, cards, and actions. Mana is the foundation for all of these resources to not just exist, but exist efficiently. So, instead of figuring out the balance of channeling to damage output, I think what needs to be figured out is mana spent vs. damage output, and then working your way from there. If I spend more mana and do less damage, I am effectively losing the game, and the same goes for the opponent.
Interesting discussion!
Oh, and it is Quoted for Truth!