Every resource I've seen and person I've talked to has said that putting low voices, usually trombones, in close spacing anywhere below C4 is to be avoided at all costs, due to muddiness. As I arrange, though, this rule is causing me some grief, as it forces me to treat the 2nd trombone as a chord-root machine, or to put the 1st trombone up into the higher part of its practical range, crossing higher than the alto voices. Not only that, but in band arrangements, making the baritone double the tuba at the octave would result in very close spacing anyway, and often between the two trombones!
Furthermore, I've recently seen band arrangements of considerable age and popularity (as in, played week in and week out every school year for no less than 80 years) that do use close voicing below C4. It's never long chords or extended passages, but they're there: Bb3 and G3, C4 and Ab4, and even (I think I saw it) Bb3 and G3!
This leads me to question: Is close voicing really so bad? If not, where's the limit? I'd never make a chord at F2 and A2, for example, and that for any instrument. If it is to be avoided like the plague, how am I supposed to handle voice leading?