No. What the trainer told you is one of the many common misconceptions surrounding the use of the ergometer. That adjustment, known as a damper, does not set the resistance for the machine. If you take a close look at the damper while you move it, you'll see that when you set it to a low number, the opening on the damper gets smaller. And, when you set it to a higher number, the opening increases. The damper actually controls the flow of air into the flywheel cage. The higher the number, the faster the flywheel will slow down after each stroke. That means it will take more work to spin the flywheel on the next stroke. The opposite is true for lower numbers.
From the Concept2 web site, Damper Setting 101
“Damper Setting is Not…
Many people confuse damper setting with intensity level or resistance.
Instead, the intensity of your workout is controlled by how much you
use your legs, back and arms to move the handle—in other words, how
hard you pull. This is true regardless of where the damper lever is
set: the harder you pull, the more resistance you will feel. Because
our indoor rowers use wind resistance (which is generated by the
spinning flywheel), the faster you get the wheel spinning, the more
resistance there will be. Think about rowing on the water. Regardless
of whether you are rowing in a sleek racing shell, or in a big, slow
row boat, you will need to increase your intensity and apply more
force to make either boat go faster. The difference is in how it feels
to make the different boats go fast. Making a sleek boat go fast
requires you to apply your force more quickly. Making the slow boat go
fast also requires more force, but the speed at which you apply the
force will be slower over the course of the rowing stroke. At a damper
setting of 1–4, the indoor rower feels like a sleek racing shell; at
the higher numbers, the indoor rower feels like a slow row boat.
Regardless of the setting, you will need to increase your effort to
increase your intensity.”
And, from Debunking the Myths: Damper Setting, Stroke Rate and Intensity,
“Myth: A damper setting of 10 will give you the best workout.
Reality: A damper setting of 3–5 is likely to give you the best
workout. Too often, we see indoor rowers set at 10, because the
athlete thinks that a higher number must be more challenging (or will
reward them with a better time). The real challenge is to accelerate
the flywheel at a lower damper setting, where power must be applied
such as in a sleek, fast, rowing shell. A damper setting of 10 is more
like a slow heavy rowboat—still a workout, but more about strength
than cardiovascular fitness. Keep in mind: the best rowers in the
world who compete at the Olympics, do not row competitively at a 10!
Emulate them; aim for 3 to 5”
So, to answer your question, if you are a beginner on the rower, I would set the damper to the recommended low setting of 3-5. If after a while, you need a challenge, experiment with a higher setting. But, I would not recommend going too high with the setting as this typically leads to bad form and a potential for injury.