After checking the factory manual it is easy to see that when you adjust the valves on a MB diesel you do indeed change the compression on the spring, but it is already compressed quite a bit when you install the valve spring cap, so adjusting the valves will change the springs compression very little. Probably so nil as to be impossible to measure.
If you have the shop manual for the 615,616and 617.91 engines a picture of the valve and spring assembly is on page 05.4-250/5.
Bill Lewallen