Nitrate samples are cheap. I would do all three. Pull a couple samples from right in the slot. Then pull another couple samples from in between the row. And then another mixing one probe from the slot and 4-5 in between. I would use a 12" probe. If nothing else you will have some interesting data. 

Other things that could alter readings: 
Was it corn last year? What was the yield? There could be some residual N leftover. 
When was the NH3 applied? How much rain has the field received since application? If it was earlier in the fall most of it will be in the nitrate form. 

It won't be %100 accurate, but you would have a much better idea what to expect.