A while ago I did an article for "Import Car" magazine: http://www.continentalimports.com/ser_ic4232.html
. Ihave been aligning MBs for near thirty years and had never used a spreader bar. Because of a thread that took place at the time on this forum, I decided to get a spreader bar and see what difference it made. Much of my results are explained in the article, but basically when placed at the front spreading the leading edges, the cars have had anywhere from .3 to 1.0mm toe difference. The standard range of toe setting is 2-3mm.
The main reason I did this test was to try and find out why I had no problems with tire wear in the thousands of alignments I had done. The answer lies in a number of things. First from a number of tests the use of the bar makes negligible difference in the readings. Second I have always spread the the wheels from both the front and rear manually (I am 6' 4") to evaluate play in the steering linkage. I do not accept play of more than 1-2mm. Also I always load the suspension to see the difference in toe to affect my settings. And lastly I believe that when doing alignments to "do no harm". By this I mean that I use the sum total of driveability (do a road test first), current or past tire wear, and the specs either set by MB or modified by experience to decide on what to set the car.