Well, if the road has a high crown, then it may try to fall off the crown, or go up the crown, but just a little. On most any interstate, it should go straight as they usually don't have high crowns.
On an interstate my 500 will go untouched for maybe 1/4 mile or more. My 300D on the other hand will put you in the ditch if you let go of it. It is only used for local travel so I haven't tried to fix it. I guess I should, but it doesn't wear the tires unevenly or anything so I just leave it alone.

The steering box has a place that is centered in its travel and should be at exactly that spot when going straight. That is what the alignment tool is for. If it is not adjusted to that spot, it will pull and or drive funny, for instance, it may be more sensitive when starting a left or right turn. I will see if I can find more information about this subject. Someone with a service manual should be able to look it up for us.

Paul

__________________
84 500 SEL (307,xxx miles)