I installed OpenCart 1.5.1.3 (Dutch language extension) and created
- a geo zone 'BE BTW' including all zones of Belgium
- a tax rate 'BTW 21%' (21; percentage; all customer groups; zone 'BE BTW')
- a tax class 'BE 21%' including 1 rule: 'BTW 21%' based on store address
- a product 'x' within tax class 'BE 21%'; price: 25 ex. tax
Our store has following settings:
- country: Belgium
- Display Prices With Tax: Yes
- Use Store Tax Address: Shipping Address
- Use CustomerTax Address: Shipping Address
and the price of 'x' on the web page is still listed without tax (€ 25.00 ex. tax)

Have I forgotten something?
Best regards - Joris M