What you say and what you show should always go together 100% .So when you’re not talking about the slide, it shouldn’t be visible. I always switch off the display when I’m talking about something that has nothing to do with the slide. If people are busy looking at the slide, they aren’t listening to what you’re saying. It’s better to use the B-key to return to a black screen or replace the slide with some form of ‘wallpaper ’such as a company logo.