I try to use both sides, but I too found that I use one side at a time, and don't use the back until I'm done with the front. When I'm flipping through the cards looking at NAs I find that I'm likely to miss something if it's on the back of the card. After I've completed everything on the front, the I move to the back.

For calendar cards I use the weekly diyplanner cards which I prefer to print duplex so that they don't take up too much space. I do the same thing with contact cards.

I've found printing duplex on cards a dubious affair with some printers doing fine on the first pass but choking on the 2nd pass. Usually printing on 8.5 x 11 card stock and cutting works however.