Most auto GPSes don't really have a fixed "start point", just a destination. They will then give you directions from your current location (wherever that is), to your destination. This way if you take a detour you don't need to create a new route - it will automatically generate one for you. And also for this reason, there isn't a "reverse course".
If you want you can disable the GPS and use iFly Streets as a simple map, with no turn-by-turn directions. Touch Menu / My Route, then click the Flag icon on the "Current GPS Position" icon - this will prompt you to disable the GPS and enter a start point. Now it's sort of like using a paper map, your route won't change, and you won't be given any turn by turn directions.
Another alternative you might consider is creating "Favorites" - after you search for an address you are shown a map to confirm it looks correct, select the "more" button on the map, then select "Add to Favorites". Now next time you need to start a route, simply select that address from your favorites list.
Hope this helps.
Walter