I replaced my drivers side one about 30K miles ago, (Have 139K on the ST now). I did not replace the passenger side, and it is still having no issues. For piece of mind, you can replace both, but you dont have to.
Hard to explain, but I am sure others will agree that you kind of get to know the sounds and feel of your vehicle, and I notice the differences right away when something starts draging or making noise. If you are attentive to incorrect feels on the steering, or noises while making sharp turns, you will probably know when the other bearing is going bad. Like I said, I am at 30K more miles on my other one, and still doing fine.