Cliff Atkinson shares on his Beyond Bullets blog that recent research shows better retention of material when slides are presented with a full sentence as the title rather than a word or short phrase.
An abstract of the journal article is here, and more information about this and related research is here.
If you'd like to explore how to make your PowerPoint presentations more effective, I recommend Cliff's book, Beyond Bullet Points.
I know I'm a bit of a broken record about this, but your PowerPoint should enhance your presentation, not be your presentation. The recent research just adds more support to the assertion that the default design settings in PowerPoint are not the most effective for communicating your message.