It all depends on the specific type of bloom boosters you are useing. Typically (depending on nutrients) you want to start as soon a they show sex or 1-2 weeks after. If you want some more detailed info on this check out some of the article here
I wait till I see the pistols 1st popping out, then start. I am using "Nirvana's" bloom enhancer. Been doing it this way for 11+ yr's & hadn't had a bit of problems. You just want to make sure to stop once they start to bulk up! And "always" make sure you have a "fan" or some kind of good air flow in your flowering/grow room at all times, so it can dry the spray up, so it just don't sit there & cause "mold" issues or anything is all..