Video Is Not 72 Dots per Inch
There is a myth in video graphic design: Because some older computer displays used 72 pixels per inch, all video created on a computer must be at this resolution. This is not true or necessary. The dimensions of a video image are dependent only on the number of horizontal and vertical pixels used in the image. Pixel dimensions alone determine the resolution of a video image. You can easily test this yourself by creating two 720 x 480 images in a still graphics program, setting one image to a resolution of 300 dots per inch (dpi) and the other to 72 dpi. Import both images into Final Cut Pro and compare the two. They are absolutely identical. This is because video editing software does not use the dpi setting of a graphics image.
Even though the dpi setting for your graphics is irrelevant for working with video, keep in mind that many people may still adhere to a policy that graphics for video must be 72 dpi. To avoid confusion with other graphic designers, you can leave your video graphics at 72 dpi. Just know that there is nothing special about this setting.
From the FCP User Manual