spraying the leaves of seedlings or plants in a veg state is fine. lots of growers even add nutes or micro nutes to make a foliar spray. If you are a new grower, spraying can give you peace of mind as well.... I know when I started growing I always wanted to be doing something with my plants.
Once your plants are on 12/12 and in flower I wouldnt spray because it may increase the chance of budrot or molds..... when your plants are in bloom you want the humidity to be rather low. but in veg, a little bit of humidity is a good thing.
hope this helps.
JJB