The vast majority of buses stop at the location of the physical bus stop sign, which is where the bus stop should be marked on the map, and where passengers would be waiting. All of the stop_positions marked on the map that I've seen in the Denver/Boulder area are simply on the road at the closest point to the bus stop, and so can be deduced automatically based on the location of the bus stop, so I don't think that including them in a route relation adds any detail.