Seems like opinions vary. On 30" rows for example....some the long rows to just meet the last end row. It looks cool in pictures but in reality those few plants are probably getting pushed over by the corn head or at least not ideally feeding. Personally, anything 6 to 15" or 20" seems good in 30" rows. Some go all the way to a 30" gap thinking hey it's that between rows when I run down them, why not next to my end row. 

But yes in an impossible game of perfection at ever-increasing speeds I'd say 10" is my sweet spot on 30's