One possible downside... all the 3rd party insurances we've gotten require getting the insurance within 'x' (like 14-21) days of paying the original deposit in order to cover pre-existing conditions. So while the deposit may not be a big chunk of change, the insurance for multiple people could be. AND, if someone changes, you can't transfer insurance, so (assuming you'd want insurance), you'd have to purchase it for the new person ("throwing away" the money from the person not going on the trip).
That being said, we usually book 12-15 months out. We actually have Harmony booked for July 2024 and we made that reservation in February.