My philosophy: if I'm pedaling slowly because it's difficult, I shift to an easier gear. I like to keep my rpm's at about 90 rpm. If I'm pedaling really fast and I feel I need more resistance, I shift to a more difficult gear.

I start in a harder gear when prepping to go up a hill, and I shift into the next easier gear when my pedaling is in danger of slowing down due to difficulty.

Just do what you need to do.