I ran a chassis dyno facility some time ago. We would try to put the car into a higher gear looking for 1:1 ratio, plus as a side perk allowing a nice long pull for a more readable & repeatable graph. Often times 3rd in a four speed car, or fourth in a five speed etc. You get the car rolling - then short shift it up to a higher gear, roll back to say 2000 RPM then let 'er eat to redline. The most important thing is to make it repeatable so you can use the dyno information as a tuning tool. Each run can be reliably compared as you make changes to the car set up. That way you can document the gains or loses.
In an automatic you can normally use "2" with finesse. You don't need or want it to shift. Its just a graph of engine power in any gear, not a top speed run. However using a higher gear allows for the better repeatable graphs.
