That depends on a lot of other factors.
There are cases where adding a dielectric union can make a problem worse.
Here's some variables
1. what's going to be in water that's in the pipe?
The more conductive and the more acidic the water, more likely you will get "electro plating" type corrosion.
2. Temperature difference between the copper to steel from the steel to copper joints? The two joints have to be looked at together since it's essentially a thermocouple that can create a voltage due to what's called the "work function"
The problem is that just adding a dielectric union can leave a piece floating electrically and that can cause even more voltage across the dielectric union, which can then cause even more corrosion.
more theory for those that are curious about how thermocouples and thermopiles(multiple thermocouples in series) work.