Sales tax varies by state but I know that here (WI) we must charge sales tax on anything we physically sell to a client. However, we never "sell" a disk to a client containing their files. The files are always available on our server or the clients'. We do sometimes give a gift of a disk containing backups of their files though.
When I first registered the business, I signed up for a seller's permit because I thought I had to charge sales tax. After talking to an accountant and finding ways around it, I was able have the seller's permit removed (and the quarterly estimated sales tax payments
).
The best money spent is on your accountant.