The two interact.
So, get solidly on a satellite,
Then you measure and change the declination a little,
That screws up your elevation, so you adjust only the elevation to bring you back onto the bird.
Then, you can run the arc again to see how well you track.
This is only one component of a comprehensive retracking, but it answers the question asked.
For a fuller discussion, I'd refer you to the
GeoOrbit site.
Near the bottom of that page is a graphic which should help you understand what to do, depending on your tracking error.
Here on the site is more
info on declination.
The chart at the
bottom of this page is the
only one I'd ever use:
Modified Polar Mount Tracking Angles
edit:
In a perfect world, you measure and set your declination ONCE, and
leave it alone!
Only if you see a tracking error as described above, might you consider changing it.