I suppose the final answer lies in what sort of work you do on the 24bit files, but the short and simple for me is, "Keep as much precision for as long as you can."
If you edit in ProTools and, for instance, adjust levels upwards to any degree (including any sort of EQ), you'll benefit from the "extra" precision. This sounds deceptively simple, but if you "turn up" a 16 bit file 6dB, you have, in effect, truncated the resolution to 15 bits.
By the same token, if you turn levels down 6dB (and you're not otherwise truncating and dithering), you may well take advantage of the resultant 17 bit word width.
Hope I'm not confusing you.