I don't always wash styrene kits; when I was a kid (b. 1964) and started building models, I never heard of the idea and never would have thought of it.
I first learned of the idea when dealing with resin kits. The release agents used by many makers of resin kits, particular in past years, left a lot of residue on the parts. It was common among builders of such kits to wash the parts to remove the residue.
I've read that in the early days of injection-molded styrene, some companies' kits sometimes had some mold-release residue on them, too. But like I said, I never washed kits back then, and I never noticed any issues. But I was a kid, too, and happy with my results no matter what. I left the hobby when I graduated high school, and by then, still hadn't gotten to the point where I'd notice that much, if there was a positive effect to washing parts or not.
Fast-forward 15 years or so and coming back into the hobby. I read about washing kits, as I said above, in the context of resin kits. And I learned that some modelers wash their styrene, too.
Sometimes I do, sometimes I don't I haven't noticed an effect, either way. Sometimes I do it more as a meditative step before actual assembly, another point, after looking at the kit in a catalog, and thinking how much fun it would be to build it; after looking at the box, and thinking how much fun it would be to build it; and after opening the box and looking at the sprues, and thinking how much fun it will be to build it. Washing the parts is one more step to think about the build.
Now, I opened a kit last night and just sat down and started assembling, without cleaning the parts. I've had 20 years to think about that particular kit, so I didn't need more time to reflect on it.
When I do clean kit parts, I use a glass baking dish big enough to fit the sprues, warm water, and a couple of drops of Super Clean. I used to use dishwashing liquid, but Super Clean is a more effective de-greaser. That's the key-the cleaning agent should be a de-greaser.
Best regards,
Brad