Yes Steve, it is a generalization and indeed has some basis in science. But it is most often used as a generalization to show that a fire which burns and spreads in some other direction is somehow "unnatural," without regard to an appropriate analysis of the fire behavior.

The "Old Wive's Tale' is the belief that a fire which does not burn up and out must be an indication of arson.

Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]