An alternative with frame bitrates which are even better controlled is to use -Vx -b n -B N -F, for instance -V5 -b128 -B160 -F for the 128 kbps quality (definitely only 128 and 160 kbps frames), or -V0 -b 256 -B 256 -F for the 256 kbps quality (only 256 kbps frames).
streaming my FLAC library from my home, transcoded to MP3, to my office.
What I meant was, depending on how this streaming is being accomplished, ABR might not make a difference.The streaming software or gear that you're using is either1. taking the files "raw" and pumping them through the network—that is, if one file is 320 kbps CBR and the next is ~160 kbps VBR, that's exactly what's going out over the network, so on the receiving end you see the bitrate changing
2. transcoding the files as it reads them—that is, no matter what format they are on disk, they get decoded and re-encoded with consistent parameters (that you configured in the streaming software) so what goes out over the network doesn't fluctuate.
If it's #2, then it doesn't matter whether you are making the files be ABR or CBR on disk before they're streamed.