It depends on a few things to determine if you need to extend the cross-sections or not.
How much area are you missing?
Are you doing a steady or unsteady flow analysis.
Do you need to generate a floodplain?
If the area you are missing is ineffective flow and you are doing a steady analysis then the results probably won’t change much, except for the added wetted perimeter.
It is so easy to do with GeoRAS or other methods that there is little reason for it not to be done. I would recommend to always extend cross-sections as that is proper modeling.