When I first leased on with Daily Express, I picked up a load to be delivered to a mining operation in western PA. I found the place, thanks to a local guy who happened to be going right by it. That was in the days when we used to use CB radios. Remember them?
Anyway, I woke up the next morning to a guy knocking on the door. He asked if I wanted a cup of coffee. I said sure. He said it's black, that's all I have. Beggars can't be choosers. I felt bad taking coffee from his thermos, I thought he was brewing a batch in the office. I drank it gratefully. To this day, I can drink it any way it's served.
Years ago my sister said her husband got mad when she washed out his coffee mug.
Apparently it's a Navy thing.
Navy guys, according to her, never wash out their coffee cups because they think that brown crust that builds up adds flavor to the coffee.
They drink it black. No sugar or cream. Therefore the crust isn't going to attract bugs, or get nasty when the dairy stuff spoils.
I don't use sugar but I use half & half.
I do get a coffee crust buildup in the mug, even though I rinse it out and let it dry. But I'm my case, my hands are too big to scrub out the inside of the mug, my fingertips can barely pin the sponge against the bottom of the mug which is just a typical 20 oz tumbler type thing. Stainless steel.
But occasionally I'll use the handle of a spatula or big spoon or something & scrub it out clean, and I think the coffee tastes better out of a clean mug.
Just like how every truck driver I've ever talked to on the subject, says (insert name of each guy's preferred coffee source truck stop) has the best coffee because they keep their equipment clean, when the gas stations run by penny pinchers neglect such daily tasks and they end up serving bitter coffee as a result.
I'm in the latter camp. Coffee is better out of a clean mug, from clean equipment.