If a DRW is sagging on one side due to fuel weight, there is something wrong with the suspension if you ask me. It should not even deflect.
"It does that," or, "That's how they are," would not be an acceptable answer to me. Shimming the suspension would also not be an acceptable solution to me.
The springs are compressed on one side for some reason, be that because of some ~1000lb weight differential from side to side, or because the springs themselves have failed. Either way I would want to know the cause.