In preparing the trip report, I use GMap "Draw and Save" to mark the route. Just click..click..click along the highways. Label important junctions. A practical scale is within 1 km in high interest areas such as Death Valley. Zoom in till the scale bar in the lower right corner shows 1 km. Then in points such that your line is within 1 km of the road line on the map.
In areas of little interest, such as vast stretches of Interstate I-5, a scale of 2 km or 5 km is fine, but zoom in to position the key junctions exactly.
In general, you just want waypoints for where you drove the car. Hiking waypoints are best handled as a separate trip report.
In your main report, to make it easy to edit the waypoints later, I typically label significant towns and highway junctions.
Having a lot of waypoints makes updates slow because it has to build the map each time. In order to edit big trips more quickly, one can temporarily move most of the waypoints to the edit notes field, and thus avoid processing them every time you do an update. Then at the end cut and paste all the waypoints back into the report.