I've read the 1/4 tank theory as well. One source said it's better for the fuel pump and filter, leaving the sediments at the bottom of the tank to be cleaned out by hand.

Another source (my car's manual) said its bad for the cats to go below 1/4 tank, and the gaslight in my car actually comes on around 1/4 tank.

Personally I always had more of a problem with having too little gas than "enough" in my car.

-Daver