Generally speaking, if you are selling tangible goods in your state you need to pay sales tax. What is a good? Whether or not a specific tangible good is taxed is unique for every state. As an independent contractor, you should be knowledgable of the sale tax laws for the goods you are selling in the states you are selling them.
A good example is that of art or music. If your client orders a printed drawing from you for example, the drawing is a tangible good and thus taxed. If that drawing, however, is delivered digitally, (through email, Dropbox, etc.) it may or may not be taxed depending on state level laws. A digitally delivered asset falls under the category of digital service and the rules vary vastly state to state.