Lamtec Corporation vs. Department of Revenue, State of Washington. In a recent ruling by the Washington State Supreme Court, a lower court's ruling was upheld that found a manufacturer's activities in the State created substantial enough nexus to subject them to the Business + Occupation (B+O) tax despite having no physical office or full time employees based in the state.
The State suggests that nexus was established by the regular and recurring visits of the manufacturer's personnel to various clients in the State. The State determined that the presence of these personnel was instrumental in the manufacturer's ability to maintain a market in the State. Said differently, without the employee visits the manufacturer would not have been able to service and retain their clients.
While this example is tied to Washington's B+O tax, I've seen this same scenario play itself out in multiple states for sales tax purposes. The challenging part is that there's no black and white rule related to the number of visits and the type of visits that will lead to sales tax nexus. Suffice it to say that if you have ANY employee visiting a state for ANY reason, your radar should go off. These visits could be sales focused, a trade show or service-oriented (as in the example above).