If you are printing your PPT slides in black & white, for the best quality you should set your printing quality to black & white.
To do this:
File Print - bottom left see Color/Grayscale, drop down menu, choose Pure Black & White for best copies.
The default is set at Color. You have a choice of Color/Grayscale or Pure black & white.
In Microsoft 2007 instead of File you press the Office Button which is located at the top left-hand corner of the screen.