Many dulcimers are made with hardwoods rather than the softwood soundboard in guitars. Hardwoods tend to shrink and expand less with changes in humidity. That is why many people get away with not humidifying their dulcimers.
With a spruce top, well, consider whether how much the air dries in winter. People who live further north here in Michigan get more dry winters and may need humidifiers. Not so much here in Detroit.
I believe Foldcraft used to sell dulcimer humidifiers. Haven't looked for a while.