The reason the sway bar is set to 10 degrees up and the correct length link is important are really the same. The lever arm for the sway bar is reduced by the cosine of the angle between the sway bar arm and level (I know everybody hates math). So if the arm is at 35 degrees up the sway bar strength or lever arm is 80% reduced. The rear bar is already pretty small so it feels like allot when you cut it further. This is why the proper length link has an effect on how it drives.
The front is really interesting. If you set the sway bar level at mid travel it has maximum leverage when the sway bar is at rest doing nothing which is not what the factory intended. The factory setup puts the max leverage on the inside wheel when the Jeep leans enough so the inside axle drops enough to counter the 10 degree offset. This means several things. First the effective leverage on the axle in a turn is more on the inside wheel so the outside spring which is already compressed does not have it's spring rate increased as much. More of the sway bar torque is effectively applied to the inside wheel in a turn. This helps keep the ride reasonable with such a large bar. This also means that the sway bar response is non-linear. As you lean into a turn it becomes more effective until you reach that 10 degrees so as you begin to lean into a turn the sway bar rate is actually increasing not only based on increasing twist but based on the leverage difference inside and out. At 10 degrees the sway bar levels off and at this point the leverage reduces as the angles become extreme, like off-road when you are flexing.
If you bias the bar down the outside wheel would see more effective leverage and be biased in a turn increasing the spring rate on an already compressed spring and reducing the lift on the inside wheel in comparison, exactly what you don't want.
Following the advice given by
@jjvw of level at mid travel isn't necessarily bad it just puts the sweet spot at an arbitrary point that might not have much use when you are driving.
The factory could have designed and positioned the joints to have the bar sit at rest wherever they wanted, just thought this would be a more helpful explanation than "just set it at 10."