Odyssey just posted about words on presentation slides.
My practice is to have a short declarative title on each slide that summarizes the point of whatever graphical shitte is on the slide. This way if someone loses the train of a slide, they can at least just read the title and get enough to keep following along the talk as a whole. The worst thing is to have someone lose concentration for a single slide and then be in a position where the entire rest of the talk becomes incomprehensible.
Incidentally, this is why it is important to repeat key shit a lot during a talk. If you as the speaker perceive yourself as being painfully repetitive, then you are probably repeating key points the correct amount.