I thought the original topic referred to washing the outside of the engine. But, my take on the inside is "leave it cleaner than it was when you got on it." I tend to work road jobs where we just swap off locomotives, or get/deliver them to a shop. Engines on today's railroad are generally so much cleaner than 20-30 years ago that there is really no comparison. I've gotten on engines that you almost had to shovel the trash out to get to the control stand. Even the outlying point "local" where the same crew kept the same engine for a week, was nasty. They would "live" with the trash rather than take it off, and it would build up for a week! Of course, back then, we had laborers whose job consisted of cleaning up after us.
So, we cut off jobs as cost saving measures and now the cleaning is left up to us.
Another point: I got wrote up a couple of years ago at a final terminal for "trash" on an engine. The conductor and I had hauled off three bags of trash when we got on the train at our on-duty point. When we got off, he left a newspaper in his seat and a trainmaster got up, saw the paper and wrote us both up for trash on the engine.