/diy/ - Do It Yourself

Are there any rules for choosing frequency bands in a vocoder?
I've settled on the circuit topology I'd like to use, but I haven't yet decided what frequencies to use.
Is the specific frequency selection actually critical, or is it mainly just important to have many equally spaced frequencies throughout the audio spectrum?
I've also noticed that no vocoder designs use frequencies closer than 1/3 of an octave.
Is that just because audio quality increase is minimal compared to the increased filter roll-off required?