A 5% loss of image around the edges won't noticably affect the image aspect ratio. In many cases it's a reasonable compromise.
Unfortunately, many video sources (not DVDs) assume that the viewer's screen has significant overscan. Many TV channels include junk at the edges of the image: some programs have timing and other video control signals visible there. Some cable TV boxes don't provide chroma for the full width -- the edges of the image are grey instead of colored. If those kinds of anomolies are too distracting for you, a small amount of overscan is a very good idea.
The standard reason for having overscan is to compensate for future drifts in the video retrace circuitry. If the image originally exactly filled the screen and if future drifts were to cause the picture to shrink, you'd start seeing dark borders.
Contrarily, if you have the set adjusted so even the corners of the image are visible, there will be slight black boarders at the edges. CRT phosphors do fade with use. If the image gradually expands, those edges will be slightly brighter. This could be quite distracting.
Personally, I dislike the idea of having parts of the image hidden from me: I prefer slight dark unused boarders on the CRT. I use a computer monitor for direct viewing of video sources, though, so size adjustments are trivial.
I hope these considerations help you decide what's appropriate.