Weather or not you charge sales tax depends on if you have nexus in that state. Nexus is a term that is roughly translated as "business presence". My business is in Alabama and anything I sell in Alabama (unless it is to be resold or is sold to a tax exempt organization) needs to have sales tax put on the invoice.
I sell the same product to Indiana, where I also have an office, and it is also taxable.
I sell the same item to a customer in Oregon, where I not only don't have a business presence in Oregon, I have never even been there, and the item is not taxable on your invoice.
NOTE: The Oregon Sale
IS TAXABLE, its just that you do not have to charge sales tax on your invoice. In this case, it is the duty of the buyer to self report and self pay the Use taxes. This is why you may have heard the term "Sales and Use Tax".
Also, you do not in the strict sense "pay" for sales taxes. You collect them as an agent for the states that you have nexus in, then remit those collections to the state on a timely basis. If you run into financial problems and eventually a bankruptcy hearing, these liabilities take precedence over most others, as the courts have ruled that if you did not remit these taxes to the state(s), then you have a breach of fiduciary responsibility (which you assumed when you got a seller's license for that state) and you can be held personally liable for these taxes, whereas you can go bankrupt on a lot of other stuff.
Sorry for the long post, but in a previous life I used to be an accountant and this is one area I knew a lot about and still do. If you have any questions, let me know.
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