If by "rocker" you mean "Circle Pad", and "cross key" you mean "D-pad":
Some 3DS games allow both to be used for the same thing, though D-pad is obviously more limited since it's only 8 directions. Other games like Smash Bros. use the D-pad for non-primary functions, e.g. taunting.
For DS games, the circle pad acts like the D-pad, since there was no circle pad on the DS.