Spraying out of fear/just in case is not really the best idea.
Usually after the first few white flies arrive, their predators will arrive 4-5 days later and balance each other out. Spraying not only kills bad bugs, but good bugs too, creating an imbalance that will then be used by nastier opportunistic bugs that then become a real problem. Unless there are great clouds of white flies, let the natural ecology of your yard sort itself out and find its equilibrium. Once you spray for one thing, something else takes over, then you spray for that , then something else takes over, then you spray for that, etc etc etc. in a never ending revolving door of spraying. Far better to let nature take care of itself. A few white flies are not going to create havoc, just wait for the wasps etc to find them and in a week or so, you will be wondering where did all the white flies go. Besides, spraying week old seedlings is like washing babies in Napalm. Don't forget, you will be consuming these plants, so do you also want to be consuming neem? The evidence is stacking up that it may not be quite as safe as suggested. Far easier to let the natural ecology of your yard find its balance and pests will never be a real problem, as there will always be another insect waiting to eat them. Once you break the chain of the natural cycle by spraying, this opens the door for something else to run rampant through your yard due to the balance having been disrupted. Interestingly, in my country, it is illegal to use neem on any plant intended for human consumption, which makes me think how safe is it really? I guess the safest is not to ever use it and keep whatever you are going to ingest as clean and chemical free as possible, including from a supposed natural product such as neem. You are what you inhale!
Hope this helps,.........
Organoman.