I've written about this before, and I am sure I will write about this again. It keeps coming up, both with my own projects and with those I consult on. I think it is really simple and it is based on both experience and common sense.
It is my belief that there is only one chance to show a script where it will have real impact -- and that is when it can be portrayed as "inevitable". That is usually when there are both talent and finance commitments -- the two components that make a dream real for the industry.
The ideal time to submit a project is when there is enough in place to create the perception of inevitability but also room to bring in others to enhance a deal. Why not make things better if you can?
The folks you are submitting to, need to both cover their ass (i.e. not look foolish) and be urged to act. They need to be able to both visualize your dream, and still have room to add theirs on to it.