When you read labels it's best to read all the words.  It's says it's best, not required.  The bigger the corn the less surface contact you get with the dual.  And have been stuck spraying that height more than once and things were fine.  I'd much rather keep weeds at bay because they'll ding the corn yield way more than any over the top chem.  

Also at 18" do you not have chemical on now?  It'll be a foot taller in a week.