Most of the advice I've seen for jump starting cars instructs to connect the black wire to a bare piece of metal on the car with the dead battery. However, I've always just hooked up both poles to the corresponding poles on the other car's battery. I've never experienced any negative consequences, and it has always worked fine.
Out of curiosity, does it matter, and if so, why?
From an electrical perspective, it doesn't matter.
However, a lead-acid battery that is charging or discharging rapidly will give off hydrogen, which is highly explosive. Since you generally make the ground connection last, there's a good chance that you'll get a spark, which is enough to ignite the hydrogen.
So while it's unlikely that you'll have anything explode, under extreme conditions it's possible. Making the ground connection away from the battery eliminates the possibility.
Chris makes a very good point regarding the hydrogen given off by the battery. There's also another reason, especially on older cars. Ground straps to the engine on most cars are notorious for corroding, so by hooking up the negative terminal to the engine you'll increase your chance of getting the maximum current flow when you're trying to crank the engine.
The goal is to start the car (specifically spin the starter motor) not charge the "dead" battery. The good battery can be wired to provide power just to the starter motor since the starter motor is grounded to the chassis. If you connect pos-pos and neg-neg, the “dead” battery begins rapidly charging and Hydrogen gas is produced as a surge of electrons enters the dead battery, too rapidly for the dead battery to capture and store. You will not charge the dead battery if you connect the good battery negative clip to the chassis.
(I am not an electrical engineer (I'm a food engineer), so I may be wrong, but this concept has bothered me and I’ve never seen a decent answer, but I think this is the briefest and fullest true explanation, I hope!)
You run the risk of damaging the alternator and/or computer ECU on the Donor/Booster car, when connecting directly to the negative of the dead car.
