SH- Wait- with steeper slopes, you don't need as much bass extension. With a less steep slope, you need more.

But I do agree with the "rule of thumb" that dictates setting the crossover a half octave to a full octave above the -3 dB point of the speakers specifically such that the natural roll off of the speaker doesn't "add" to the imposed slope from the crossover. I personally believe that a lot of people have their crossovers set too low.