woothemes/woocommerce — pull request #1352 by patrickgarman (2012-08-07 20:45:39+00:00)
Title: Taxable Price Filter

apply_filters( 'wc_taxable_price', $price, $key, $price_includes_tax, $compound_tax )

For tax laws such as this one:
http://www.mass.gov/dor/individuals/taxpayer-help-and-resources/tax-guides/salesuse-tax-guide.html#apparel