I usually plant 20-25k.  On 30" rows.  I have also gone as low as 18k.  As long as you are getting consistent singulation, and you have plants spaced every 6-8-12 inches, it can really compensate for the lower population.  I have never seen a big advantage to more seed unless you have lots of rain, or irrigation.  If you are just a little bit drouthy, lower pops are much better.  
I don't remember but I am sure this was planted less than 25k.  And it was a good thing, because it almost burned up before it made a crop.  Made about 60bu, which is really good for our area.