I'm not sure about the US, but in Canada, you only have to charge taxes and get a GST number (tax number) if you gross more than $30,000Cdn/year. At that point, if you're selling your product to a Canadian, you have to charge GST (the country-wide tax). If you're selling to someone in your province (Canadian version of a state, if you were unaware), you also have to charge them the province tax.

For example. I'm in Ontario, Canada. If I sell my product/service to someone in California, I don't have to charge them any tax. If I sell to someone in Vancouver (in BC - not the same province as me), I just have to charge them GST. If I sell something to someone in Toronto (the same province as me), I have to charge them GST and PST (both the country and the province sales tax).

From what I understand and have experienced while living in the states, the same sort of concept applies as well, though I am not sure if there is a gross minimum or not, like there is here. For example, if you look on certain sites, you'll notice if they're based out of California, Cal. resisdents usually have to pay 8.25% tax (or whatever the tax is).
Hope that helps.