FAQ: Should I worry about nexus if all my sales into a state are "for resale?"
Yes. If you have nexus in a state, you should be collecting that state's seller's use tax for any shipments into the state, of have a good explanation why not. If you don't have that explanation, then you will be held liable for the taxes. Don't make the mistake of assuming you're off the hook because you're shipping from out of state.
The best course of action, obviously, will be to get exemption or resale certificates from your customers for the state where delivery occurs. That way, if the state catches you, you can wave the exemption certificates at the auditor and say "hah, hah, hah!"