As soon as you play with them, you void the factory warrenty on the front end suspension. If you are going to do a lift, replace them (as the lift will also void the warrenty) If you are staying stock, leave it alone, as they sag more, the dealer has to "deal" with it, as it is a warrenty issue. If it fails, they replace it.. You monkey with it, and you are on your own.
When you get gas, do the finger-on-hip measurement from side to side.. if it is dropping, send it back to the dealer...