Technically it does drive the deflection a bit harder, but we're talking a few percent, not nearly enough to cause any problems. Assuming a properly designed monitor, you won't be able to adjust it high enough to damage anything.
You can try adjusting the focus, but it's a static focus control, and you'll probably find that you can make it razor sharp in the center or at the outer edges, but the best focus overall will be a compromise. High end CRT monitors have complex dynamic focus circuits that vary the focus voltage depending on the H and V position of the beam.