This is all very true if your paper has low or no reciprocity failure.

If there is reciprocity failure than speed and/or contrast may change if you change the time.

In my professional work, I was taught to change the aperture while keeping the exposure time constant! Of course, you pick the center point with time so that lens sharpness is optimal but you have a stop or so on each side of "perfection" to work with between 8x10 and 16x20.

PE