A few days ago, I took a short side trip on SD 45 south from SD 34 to Gann Valley, before returning to continue on SD 34 west to Pierre (SD 45 was rough, anxious to get to Pierre before sundown). Gann Valley is the seat of the Buffalo County government, even though it's a tiny town (population of only 14, might be the lowest-population county seat in the U.S.) that otherwise wouldn't rate a waypoint.
Gann Valley has four intersections with SD 45: Moutton Ave., Cleveland Ave. (per ESRI WorldStreetMap, and confirmed by signage in the field), and the north and south ends of Main St. Any of these points would work for me, since I passed through all of Gann Valley before turning back north. I suggest placing a point at the Moutton Ave. intersection.