Originally Posted by RST
The more modern look (as well as antique) is to have no borders at all.
The rule I learned, way back in the day, was that it's visually pleasing to have your borders be a multiple of your block size. So if your block is 12 inches, then that suggests that a 6 inch, or 4 or 3 or 2 or a combo of those would look good, whereas a 10 inch block you go with a 2 or a 5. If I'm ever stumped for what to do for a border, and for some odd reason feel the need to border a quilt, I think of that guideline, and may or may not follow it.
The only reason I can think of for mandating wide border is that it gives you space to quilt a more elaborate pattern, if that's the look you're going for.
RST
I didn't know this was a "rule" but it is something I have always tried to use. It just seemed pleasing to the eye. If I wanted 2 borders for a quilt with 12 in blocks I would mabye use a 4 in and 8 in border. Making them different sizes was also pleasing to my eye. However, this was not a rule someone told me I had to do, just something I liked.