In my opinion you should NOT switch back and forth. IK sets the end of the IK chain as the target and calculates the positioning of the rest of the chain. In Poser the hand is the endpoint.  The translate x,y,z values are only applicable if IK is on.  IK should not change once its set if its a keyframe but the spline interpolation that poser uses by default may be slightly altering the position and giving you a bad pose. You should be able to correct it by fine tuning of the curves and maybe adding some more keyframes to keep the curve very close to your needed values.

I hope the above is clear :(