I have a select core of repeat clients who contact me in pre-production well before post sound is to be performed. For these clients I can start forming sound ideas and recording sounds based on their script if needed. Sound is art and it is best served when it is considered a part of the film process from the start. The business reality, which is perfectly acceptable and very much the norm is not formed around this ideal and most of my sound design work begins after picture lock. While the majority of independent films are made this way I can say that all of the successful filmmakers I work with do consider and act on the importance of sound from the beginning.
I do encourage filmmakers to contact me as soon as they can in the process. I am always willing to answer questions about production sound puzzles for music playback or help suggest possible sound recording methods or post production audio workflow options.