When I started to go back and build a "retro" build I was very determined to learn to retrobright and try and restore everything to its absolute best, the clean beige case with the non yellowed CD-ROM and floppy drive, etc. That attitude made me learn a whole lot but I eventually relaxed and started to appreciate the yellowing in most cases and some marks of patina. If something arrives at my hands in a certain state it doesn't bother me at all and I just do what I can to clean it up and fix it, but if I know that I accidentally scuffed or marked it further in some way it's more difficult to accept.
I made several experiments and techniques with retrobright, and every single type of different plastics eventually re-yellowed between 6 months to a year and a half. It re-yellowed to a lighter shade but remained yellowed all the same. However it does work well to make the shade uniform all over when it's very uneven, and that's probably the only scenario where I would do it nowadays. The effort and money when measured against the eventual results just aren't worth it, imo.
As said here before I also realized that there's not much of a point to try and achieve absolute perfection especially with the build's case, I barely even see it or look at it and when I do it's in a half-light spot which actually hides the patina that I was fine with anyway. I used to re-spray paint the metal parts but they will get marks anyway as you fiddle around and the cables behind it sometimes touch the paint which makes a chemical reaction that leaves marks on the painted parts. Barring extreme cases I would never have painted them but oh well. I don't think I won't repaint them for a long time, it's all in areas that I would never see without going back there with a purpose.
I even have a dark grey HP CD writer with blue buttons that clashes with a beige case, but because that case also has buttons of that exact blue color I purchased a Pentium III Intel Inside metallic and blue sticker that ties everything together and I quite like it now. Barring extreme yellowed shades, I'm not that bothered by stuff in the front panel anymore. Of course having an LG CD/DVD drive on a 486 is not ideal, so I would try and go for one of those with a blank front plate. What I would never do is use Dell black/metallic case for anything pre-WinXP for example, that I do know I couldn't accept, even if a beige case would be yellowed in comparison.
All in all with a keyboard and mouse plus a CRT that are in good condition chances are one is visually satisfied in the day to day usage, but to each their own