I would say both have benefits and drawbacks but I would say proactive. Using it reactive could risk starvation after the initial algae teardown post addition. You can generally predict when youll have algae boom aka large new addition, new tank uglies, and purchase proactively (but conservatively). Introduction slowly as the bioload increases is my motto. I do warn against dumping in CUC during the uglies as there should never be more food for the CUC than during that phase, so after it ends your CUC is in trouble.
However, the big problem i see with new reefers is thinking a clean-up crew is a cure-all. If algae is growing, I'm inclined to believe there is a nutrient issue or some sort of instability. Slow fixes will produce long-term stability and as you figure out the way you want to deal with algae, you will hone in on the method that keeps your tank the way you want it to look 24/7 without large intervention.
definitely overdid the response but too late lol