No, the templates are for exactly this.
Go to the template configuration (menu button), long click on each of the pre-configured templates and then delete them. Add a new template, which applies to all lists, with the default due date, tick the default flag.
Now whenever you add a task, a default due date will be set. If you setup multiple templates, only 1 can be default in each list - so long click on any + button to get a template list up for you to choose from.
see:
http://gottodoapp.wordpress.com/user-guide/task-templates/Hope that helps
Andy