I'll be the first to admit I'm not the expert mechanic that many of you are. But I had just installed a new (German) PS belt and both pulleys were in good shape, so I wasn't trying to compensate for a worn/glazed parts, etc. I made several incremental adjustments, tightening the belt just enough to not slip on the pulley, and no more. I recall checking the belt deflection and feeling satisfied that it was still within spec. Then I proceeded to carefully turn the engine in small increments via the PS pump. I don't see the harm in doing this.

Even if you were to go back and slacken the PS belt slightly once the valve adjusting was done, I'd still find it more convenient to do that than trying to rotate the engine at the crank throughout the whole process. Just my .02, "your mileage may vary".

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1995 E300 200k
1981 300GD unknown km