Sales tax is not charged on all items you buy. Check online to see what is, and is not, taxed in your area.
If you are actually rounding the tax rate down, make sure to remember that your estimate will be a little bit less than you will need to pay.
In doing this, you are actually multiplying the cost of your item by the decimal equivalent of 10%, which is obviously really easy! For example, . 10 times $20 equals $2.
