I'm not a lawyer (or an accountant) either. I believe the threshold varies by state but I don't know if there's anywhere that it's at a level where "normal" transient travel would expose you. I assume the biggest issue is people like consultants who may travel to a work site during the week for months at a time. (Or, as you say, someone who has a permanent address of some sort in a low/no tax state but actually lives most of the time somewhere else.)

ADDED: It appears that the threshold is essentially non-existent in many states. https://www.concur.com/newsroom/article/what-are-state-tax-i... (I think this is the solution my friend's company is actually using.)