Change Checkout to show prices before Tax

Normally, the decision to display taxed prices or untaxed prices is set in your menus/modules this option is then used on the checkout.
If the Itemid of your urls is lost, then, then default option (in the configuration , under the tab display ) is used. That could also be a way of forcing the checkout to a certain menu ( using the new option to do that in the tab display of the configuration) for which you could set this option.

Finally, you can also add at the top of the file step of the view checkout after the php tag, in the menu Display->Views, the code :
$this->params->set('price_with_tax',1); //or 0 for untaxed prices
And that will force it on the checkout page regardless of all the options.