Mercedes shows an adjustment procedure for the gear box in the repair manual(s), so I'd assume from that alone that the steering box adjustment is considered normal.
Just a couple weeks ago I had the steering box adjusted on the 300D with 285,000 miles on it. That is the original steering box and it still has adjustment left in it. Same story with the 300TD with 145,000 miles. Wish I could have gotten more out of the one on the SL, but for some reason or another it was not to be. So, I purchased a rebuilt one from
http://www.************************ for about $275.00...during one of their 15% off sales...and paid a couple hours labor to get it installed. About $450.00 for the repair as I recall.
I'll continue to adjust mine when needed and will replace when all the adjustment has been taken out.
Regards,