Before any design commission of any real relevance gets off the ground, a design proposal and design contract is usually in order. After proceeding haphazardly into design work without one, I can testify that I think I would have rather taken the time to set some parameters in writing. Really this applies in general communications about agreements, business or work. I would also venture to say that it probably benefits both parties in the long run. Why? When things are set in writing everyone knows, or should know, what to expect. If they forget they can always go back to the written contract, email or proposal and get clarification.
For the designer or developer it provides a task list or blue print that can help to keep the the project on task and narrow the scope. This is important because very often a client may have something that they would like to add in the way of work. Since you are operating as a business function this will require more time. More time should should be compensated fairly to the business, and importantly so if it takes significantly more time from other business endeavors.
The client benefits from written agreement equally since it outlines what is expected and what will be delivered. It can also provide a guide to the design or development process. If the designer forgets to add a certain module design element for example the contract can help clairify this.
This is a testimonial. For new design proposals, or any other work or agreement for that matter, at least, shoot off an email that outlines what your understanding of the project or agreement might be.
It could save you many hours of extra time and potentially a trip to court.
If you really want sound business advise you might want to consider one before going into business agreements.