No. You shouldn't have to do anything on the USPS end to get it shooting back rates that equal what you think.

I know you said you looked at the Tare weight and corrected that already, but did you change the following setting as well (in that same section):

Larger packages - added packaging percentage:weight

The default is "10:0". This means that 10% will be added to the weight of any package before being sent to the shipping rate system.

On the "Modules > Shipping" side of things, did you know that depending on what you set the following setting to ("All Packages are Machinable"), this will affect the rates returned as well? You may want to look at your setting there.

Finally, as a means of "debugging", I would display all the elements that the shipping module lets you display (weight, number of products, etc...) and then compare that weight etc.. to what you are entering manually.

That will at least tell you that if the weights do not match up, it has something to do with the way you have configured your product weights, shipping weights, etc.. or if the issue is not related to that at all.