I tend to set the start date as well for each task, although I understand that the vast majority of users only set the due date.
When I start creating a task by setting the start date in Toodledo Web and move on to the due date, the due date doesn't reflect the start date. It always shows this month's calendar around today. I think this is a little counter-intuitive. In MS Outlook, for example, as well as other mobile apps that I've used with my WM phone, when the user sets a start date for a task, the due date adjusts and automatically starts from the start date as default. It makes a lot of sense.