Do I need a dielectric union if going from polished carbon steel to normal carbon steel?

No... those are for when you have dissimilar metals... like Iron to coppor or aluminum or brass... as examples. They eliminate direct contact and the galvonic corrosion that results.

Hi bpcorinth and welcome to The Plumbing Page at AMHD.com.Hi
Smoothy nailed it! Dielectric unions are used to prevent electrolysis between two dissimilar types of metal. Good catch Smoothy! Tom