Hi there!
They say that the best time to apply preen is after weeds are actively growing but have not yet set seed, so about the time that dandelions start blooming. You also want to weed the beds thoroughly before you apply because if you disturb the soil the pre-emergent barrier will be gone. Also do it after mulching, because it won't do anything underneath mulch.
The big problem you may have, though, is I doubt the active ingredient in preen will do anything to stop maple seeds from germinating. It is mostly a grassy weed preventer, with some broadleaf weed control, but may not do anything against tree seeds. Personally I believe a good layer of mulch will do more to prevent weed seeds from sprouting than preen, plus it improves and loosens the soil making weeding a much easier chore.
Last year I used a weed killer spray with weed preventer in it, and this did a great job in my beds at keeping weeds from coming back, but I still have a lot of walnut and pine seedlings to deal with, so I don't think that spray prevented any of those.
Hope that helps, and good luck with your gardens 
Chris
