juiceme is right. There are some advantages to announcing early, such as you maybe get more feedback and you get an opportunity to improve your entry before the competition deadline.

I don't think there are any disadvantages to announcing early in fact - even if you don't have any code and only want to talk about a specification. We have an edge case of someone doing it and being unable to collaborate and becoming a competitor in a way, and you would have to start from scratch with another idea or struggle to make a 'better' implementation, but I wouldn't worry about this - our people are kind and usually willing to collaborate and give credit when due, and we have a massive wishlist so there's plenty of ideas to work on.