What you say and what you show should always go together 100%.
so when you're not talking about the slide, it should'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 with some form of wallpaper such as company logo.