Q: Should I use pressure-treated lumber for the inside of a shed I’m building?
A: Using pressure-treated lumber certainly wouldn’t harm anything, and it will always be more durable than non-treated lumber. You must used “ground-contact” PT lumber for any framing members that touch the ground. And use treated lumber for any exposed areas. Is the shed open or enclosed? If it’s an open shed, then yes, use PT lumber. If the interior is completely enclosed from the elements then PT lumber is not absolutely necessary but it’s still the superior material.