For kicks I took the lid of a yellowed Apple //e and washed it in the dishwasher with the dishes. I left the rest of the chassis unwashed. I took before and after pics.
Here is the result.
Comparing the washed lid at the sloped spot where the Apple //e is printed) with the main chassis nearby, there appears to be little or no improvement/change in the yellowing stain.
NOTE: Because the monitor usually sits on top of the cover, the flat part of the lid is usually less yellowed than the sloped part. Compare same views of before and after.