The theory here is that running grey bars instead of black gives more consistent usage of the entire screen, so that any "burn-in" is distributed across the whole screen and not focused on the pillar-boxed content area.
In practice, and according to recent research, modern plasma displays are not very susceptible to burn-in. There is some "persistence of image" if you run, say, a news ticker all day ... but that image dissipates in a short time under normal usage.
Bottom line: Do what looks best to your eye, and don't fret burn in.
Cheers,
- Shane