It should make little difference in todays modern cars with regulators that keep electrical output at constant levels of voltage in order to charge your battery with regulated current. (maybe 60amps)
But during start, your vehicle requires a massive amount of electrical current (hundreds of amps) for a very short time in order to turn the starter. It is the battery that is doing the work and not the alternator, which is why those jumper packs you can buy at the store are so effective, they are designed to put out the cold cranking amps. If you have to you can even jump another car with the jumping 'good' car not running at all.
That being said, we all do it since it makes us feel good.
Hope that helps,
Umm...I started to pass this by, but had to put my 2 cents in.
The condition of the "good" battery, and its cables and posts, is important, when qualifying these previous answers.
The "terminal" or post voltage is what the battery delivers to the cables. If the "good" battery is weak, or the terminals are resistive (corroded), the alternator output (voltage) will help hold up the voltage being delivered to the jumped vehicle.
Whenever the battery is called upon to deliver current, its terminal voltage decreases, even when another car is not being jumped. If the drain is too high, the voltage decreases dramatically. Having the alternator deliver power will help hold up the "good" battery voltage, and compensate for the assumed lower voltage of the jumped vehicle.
My vote is to have the "good" vehicle delivering alternator current to aid the battery. I doubt revving the engine helps much, though.