I noticed something with respect to tags that I am trying to hunt down. This may end up making more sense on the USRP mailing list, but I figured I would try here first since tags are a GR thing.
I've noticed that if I have a simple flowgraph of a USRP block with two RX channels, if I change one of the frequencies, a tag is generated on both streams. I believe that the act of changing a frequency cause there to be dopped samples, so it would make sense that there is a tag for that stream (not to mention that the new frequency needs to be conveyed). But I am not sure why GR is generating a tag on the second stream as well. Maybe the way the USRP handles the streams it ends up dropping samples on both to keep things in sync, but I wasn't sure if GR was doing something here instead.
