The only time I include sales tax is in Memory Mates or School Picture packages... these are pre-pay events. For all others sales, I add the tax when an order is placed.
In Ontario I think its illegal to have the tax in the price already. I think you have to display the before price, and if you want to give the tax price you have to display the 3 parts. This way people cant scam or include hidden taxes.
Could be the same way in some states/other provinces.