I noticed today that after going down a hill and then coasting up that before I start pedaling again I down shift the rear cassette one gear. Is this hard on the shifter or not really as long as a guy is only shifting one gear?

Not a problem. You can also soft pedal while you change gears if you're at all unsure.

by "down" do you mean to a larger cog/easier gear? if so, just one is probably ok...but i wouldn't do more than that or leave it too long without actually pedaling and letting the chain shift. shifting that way stretches the cable, and i've actually snapped a derailleur cable (it was already worn, but still) by absent-mindedly shifting multiple gears at once while rushing to get moving after changing a flat.

go for it. we have the technology now. better yet, pedal the downhill.

The derailleur is on a spring, so it's not going to hurt anything. It is pointless, though. Besides, you should pedal in the big gear as soon as the grade starts to even out and cycle through the gears while pedaling to keep your momentum. You're losing a lot of energy by coasting while you're losing speed.