Brian right-on from my experience. I took the '91 and '95 to the local "super" tire store. Great people but couldn't get the alignment right. Repeatedly something was wrong. Burned off the outer edges brand new RE950s on both cars.
They have the latest and greatest laser racks etc .... took them to the dealer, and the first time, right. BTW, if the dealer has an old Bosch optical rack, run to a dealer that has a modern rack. Insist they use the spreader bar.
Once the alignment is right, unless you hit something you shouldn't have to align it again.
That's my expereince.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)