So NASCAR runs on enough of a variety of tracks that there are (very, very generally) three modes of running:

1) Reduce drag to the bare minimum. The cars can still grip, even when trimmed out all the way (Daytona, Talladega)

2) Downforce is king! I don't care if I'm 10MPH slower on the end of the straight if my mid-corner speed is 15 MPH higher (pretty much all "intermediate" (1-2 mile) tracks (this is the vast majority of tracks)

3) Aerodynamics are irrelevant (Short tracks like Bristol and Martinsville)

So there's not just one goal a NASCAR builder is optimizing for. You generally have to make compromises to win a championship.