[jeanne]

I don't have code like that in cart.php. The code is already set up to do this if your tax zone is set up by zip code.
I have my destination zone (that I use for taxing) set up by zip code like this:
90%
91%
92%
93%
94%
95%
960%
961%
This mod is basically giving a temporary value to the 'Default Zip Code' entry that you can set in General Settings.
So when an zip code entry is put in the shipping calculator that matches my tax zone, the tax shows up as long as the 'Default Zip Code' entry in General Settings blank. If you have any other of the default settings (zip code, state) filled in clear the entries to see if that makes a difference.