On freeways, yes. But on city streets and roads with many intersections, driveways, turn lanes, etc., I don't think that's really true.

Staying in a lane while also maintaining appropriate distances seems better than focusing on maintaining constant speed in traffic. The key is whether you can maintain a reasonable constant speed relative to traffic without changing lanes.

Basically, always driving at exactly the speed limit to the degree possible is a questionable heuristic when there are a lot of cars.