From Richmond Gear
I guess the manufactures don't know what they are talking about.

http://www.richmondgear.com/01instructions.html

BREAK IN
A new ring and pinion installation, especially a high numeric ratio with new bearings, can cause an excessive heat buildup in the rear end and cause softening of the gear teeth and bearings if a break in is not performed.

Street vehicles should be driven at normal street driving speed for approximately 10 miles, then stop and let cool for 30 minutes. Do this 2 to 3 times. Towing vehicles need approximately 200-300 miles of normal street driving before being used for towing.
On circle track race cars make approximately 6 to 8 laps at slow speed, then let cool for 30 minutes. Make 6 to 8 more laps at slow speed, then 2 to 3 laps at full speed, the let cool again for 30 minutes.
Drag cars need only an initial run-in since they are driven short distances and heat is not normally a problem with proper lube and backlash allowance.
NOTE: If after the above break in is performed, overheating of the rear end is suspected, repeat the final portion of the break in procedures.