How important is it?
I'm installing a new boiler with existing black pipe. I will have brass valves (3) and brass taco air separator. Are the dielectric unions necessary? Is this a BIG deal?
I'm not a pro. That said, I've never seen dielectric connections used to isolate brass fittings on black iron piping.
Equally anecdotal evidence is noted in the 40 year old boiler that I replaced had a mix of brass and black iron and I found very little corrosion. The local water supply is naturally soft so that may have helped. Since there is such a large amount of iron in the system I'd think that unless a really hot local cell developed the mix of brass and iron would be ok.
On the negative side was the one place I found corrosion/scale was on the dielectric unions on the water heater. The 3/4 fittings were down to about 3/8 ID. No wonder I had to crank up the water pressure to get decent system throughput after installing a PRV. Oh well, all that is gone now.
Thinking back to the bad old days offshore, we mixed brass and black iron all the time. Corrosion from the outside was the usual failure rather than internal to the system. Something about 24/7 exposure to saltwater/salt air made those system a maintenance nightmare...