The Tare weights are fine for my shipping (or at least they've been fine up to now). Here are my other settings:
Shipping/Packaging

Title Value Action
Country of Origin United States
Postal Code 99205
Enter the Maximum Package Weight you will ship 25
Package Tare Small to Medium - added percentage:weight 10:0
Larger packages - added packaging percentage:weight 10:0
Display Number of Boxes and Weight Status 0
Order Free Shipping 0 Weight Status 0
Shipping Estimator Display Settings for Shopping Cart 2
Display Order Comments on Admin Invoice 2
Display Order Comments on Admin Packing Slip 2