It's all about the roots finding what it needs at v3ish when the nodal roots take over from the seed as an energy source.  If it's warm and the root find lots of N before the seed quits sending energy, then the return should be small.  I would say it may be okay to stop.  Is your planter a multiple of your corn head and to you have a yield monitor.  You could do a whole field with 1/2 a planter and see for your self.  Or if your like me do 8 and 4 as I have a 12 row planter and 8 row head.  That would give really good results in a whole field test.