I view everything in OAR (original aspect ratio); e.g., I view 4:3 in 4:3.
For cable sources, I use the darkest side bars I can in my HD STB, but I would describe them as dark gray rather than black. If black were an option, I would use that.
IIRC, 4:3 DVD sources have black side bars.
One of the reasons I bought a microdisplay based RPTV is to view everything in OAR since they do not suffer from burn-in, and I hate stretched images.
Thanks
Bruce