If you are saying it sags after you turn off the engine I think that is normal.. unless you need new springs so the default height will be higher...
Most people don't know that manufacturers consider 50,000 miles as the length of life for springs... they should be replace or rearced to bring the suspension back to the height which will allow the proper setting of the wheel alignment...
Of course, after me saying that... many will post saying they have 750,000 miles on their MB's and the springs have not sagged at all...