For Outlook-tasks, you can leave both the start date and due date at (none). However, if you set a start date, you must set a due date. (The system defaults to a due date the same as your start date.) Once an Outlook-task becomes overdue, its color changes from black to red. You can also have multiple views in Ouitlook-task, so you can have one view for everything you might do in your entire life and another view for tasks with a start date today or earlier.
For what tasks you want to do tomorrow, next week, this month, it makes sense to have a start date set for the earliest date you can start such tasks. My advice is to assign a due date on everything. Let me give you an example of a low=priority task; On my @ Errands list right now is "Return library book" (which is due in a week), "buy yogurt and creatine from a specific grocery store". The library book incurrs a fine at $0.15 a day so no urgency. The grocery store is right beside it and I have almost a week of yogurt and creating. I put a due date on the grocery store for 5 days and the library book due in a week 2 weeks from now. the start date on both is today. These tasks have an Outlook priority setting of "Low". Before I get in my car to drive somewhere, I look at my @Errands list in Outlook to see what I can knock off. If I don't get it done by its due date, it turns red, which is my signal to re-evaluate it. This is my signal to bump it to a higher priority task or schudule it as an appointment, As another example, I have an auto-repeat task "take garbage to curb" every Tuesday. This is a high priority task as it must be done early in the morning.
Before you laugh at the trivial nature of these tasks, consider that my Outlook-GTD system looks after them completely! I don't have to devout any brain power or memory to them because I know my trusted system reminds me of such tasks exactly when I need to know.