Like all things, it depends on what you are doing, and with what.
Way, way back, like in prehistory, at least pre-internet, the mid-late 1970's, I was being taught how to build plastic model airplanes. My Grandfather taught me that with some of the smaller companies of the time, you had to wash the parts before you put them together or the glue and/or paint would not set up and just fall right off.
This is due to the mold release compound used in the plastic injection process. It makes it so the newly minted plastic sprues can pop right out of the mold. It can leave behind a bit of residue on the plastic, which causes the building/painting issues.
I remember a few kits that came from overseas that were practically dripping. The vast majority of U.S. and Japanese kits had no issues at all, and I stopped washing my plastic from them, or modern Games Workshop plastics.
Then came the metal figures of the '80s-'90s. Mostly they were fine, but on occasion I would get parts that would not hold together no matter what. I normally didn't wash metal, but X-acto'ed clean the fitting surfaces.
Then resin kits! The early ones needed a ton of trimming, then washing. Sometimes repeating the trim/wash cycle to get the parts to attach. I washed my early Forge World Baneblade thoroughly, and still had trouble with parts holding together. Then it rejected its first attempt at priming. Sand down and re-wash, then it was okay.
Modern Forge World resin is much plastic-like and is a joy to work with compared to the old days, or some other companies. I will always wash resin, or soft injection plastic figures. It just makes life so much easier.
The Emperor Protects
But You Have To Do Your Own Wash