Turn it On if you are required to display prices including tax. If set to Off then taxes will only be included in the order totals on the checkout confirmation page. Starting AbanteCart 1.2.10 Tax Inc. text will be added to taxable product's prices in listings and product page.
By default Guest customers with unknown shipping address will see Prices with Tax rate applied to default store address. Please check your Tax and Location settings if this does not work for you.