I am creating a presentation of slides to be used with a projector. Most of the slides contain lists of incomplete sentences so as to not clutter them up with too much text and overwhelm the audience. I have written the phrases without ending periods. Should they have ending periods?
I think that this is personal preference.
Periods tend to show that it is the end of the thought. I find them useful in separating out concepts. If all the concepts are related, you might leave the periods off, but really, it's more about personal preference.
The most important thing you can do, though, is be consistent. If you have one slide with periods, all your slides need to have them. Otherwise, the presence or absence of periods may become a distraction.