In jobs where I feel sure I'm doing my job well, I overtly show when I'm not busy. Walking around, playing video games, chatting, even working on my own business (with boss's permission) etc. I see other people worriedly noticing when the boss is coming or being afraid that the guard will notice they often turn up late in the morning. But it doesn't bother me at all. I'm doing what I was hired to do and there's no need to make a show of working. Sometimes coworkers see me on the computer and say "you're working hard" but I laugh and say "no, I've been pissing around all day, I'm really lazy". Of course this does depend on the job - sometime you are hired to make full use of your time, not just get things finished.

However, I have worked in places where I always felt I wasn't performing well enough. Stayed at my desk doing "things" as fast as I could and still being behind on deadlines, a couple of times by making big mistakes that meant weeks of work turned out to be useless. I'd go home drained and stressed and still not be refreshed enough to think clearly again the next morning.