In general, no. Our water meters are made of stainless steel, which is close enough to both copper and galvanized steel, on the periodic table, so there are no negative consequences of directly connecting them. In certain environments (ocean or salt spray) there will be some corrosion, so dielectric unions are advisable.

Dielectric unions are used when two dissimilar metals need to be connected. For example, copper and galvanized steel. If no dielectric union is used in this case, then there will be galvanic action and the two metals will corrode (they basically act as a battery) over time.