Jay, I just read one of your dopy questions on tw regarding those lower ball joints. I think my wife is more on the ball.
Let me ask you this....your drive shaft has several u-joints, and one fails years later...do you just change the bad one, or replace all of them at the same time?
Here's another...you blow a headlight some years down the road...now, do you just replace the bad one, or do both? It's not a trick question if that helps.
Even easier....say you just came from a shop that rotated your tires, but found one with loose lug nuts....do you tighten just that one wheel back up, or do you now check the other 3 for the same?