I think they only need to be replaced if broken or like mbpgawest says, when contact is made and the light comes on. I am pretty sure that there will be ample warning (squealing) before the sensor will make contact and the warning light will come on.
I have a broken sensor that I think resulted from the wrong pads being used. The sensor contact seemed like it was heat damaged and crumbled when changing the pads.
My 97 has two sensors not four -- one on the front right and one on the rear right. The other sensor disconnected and reconnected easily during the pad change.
I don't think they are supposed to be replaced each time you change pads unless you waited too long.
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