As a sole proprietor nothing precludes you from having a web development agreement between you and your customers. In general, it's always good to have things in writing, it will not only protect you, but it will help eliminate confusion and address issues that may arise between you and your customers down the line. If you are planning to grow, hire employees, or just want limited liability and pass through taxation benefits, than you might want to also consider forming an LLC rather than a S corp - but it really depends on your circumstances. It would not hurt to consult with an attorney...