Fungal is the only one I would say is recommended spraying for as a preventative, but even still often times it is bad management that causes many issues in regards to fungal. Proper air circulation and watering and you should really have no issue with fungal problems 95% of the time for most plant material. Fruit trees are a different story, and a few other specific cases (keep those junipers away from pear trees), but I am coming from a horticulture background in regards to landscape and food crops in ground.
As stated earlier insect pressure many times is a symptom of overall health problems. Plants can naturally withstand and deter reasonable pest pressure (otherwise plants would not live very long). If it is getting out of hand there is likely another problem that is effecting their natural ability to fend off/handle pest pressure. Again, this is from a non-bonsai perspective. So not 100% sure how much this knowledge transfers to smaller plant material that is managed in a totally different way than in ground growing and even most potted plants. But the takeaway is healthy plants can handle most pest pressure.
IPM (Integrated Pest Management) is a pretty established healthcare program model, so to contradict everything I just said there is plenty of merit to plant material being put on a program to control pests and ensure the health of a plant. But these programs are not simply "I will spray my apple tree every two weeks with a copper based fungicide". So I do not think it would hurt. You can geek out and use local resources to view growing degree days (GDD) to anticipate when pests will be present.