It would be so nice if these forums could be used to get help on issues instead of just bearing witness to struggles. After asking for help on various similar threads, I finally figure this out and am posting it to help out anyone else trying to make this modification.
I'll start by saying that I changed the core code and if someone can point out to me how this should be done using an override, it would probably be better.
I'm using version 1.5.2.0 with a custom theme based on the default that came with the install
The file to change is in your admin directory, then themes/default/template/controllers/products/prices.tpl
On line 165, find this:
<option value="{$tax_rules_group.id_tax_rules_group}" {if $product->getIdTaxRulesGroup() == $tax_rules_group.id_tax_rules_group}selected="selected"{/if} >
and change it to this:
<option value="{$tax_rules_group.id_tax_rules_group}" {if $tax_rules_group.id_tax_rules_group == #}selected="selected"{/if} >
with the # sign being the id of the tax rule that you would like to be the default.