Second to TerryZ.
If you are dosing medication, example Kent Marine's RXP, you will need to off the skimmer and take our carbon that is running.
The instruction stated clearing on most of the medication that skimmer should be off fo an hour. As for carbon, it absorb the medication and thus making ineffective. You will be throwing good money to waste.
Coming back on trace element dosing, in my case, I never off my skimmer and take out carbon. No much impact based on experience, which is purely my view.
Since dosing of trace element is a often regular task, turn off and ON your skimmer (electrical) device will shorten the life spam of the pump.