I've posted this before:  a sonar CAL program to set all the selected midi events to any chosen midi channel.   It doesn' much matter EXCEPT when you are using the PRV and viewing a controller in the controller pain:  then, you are restricted to viewing one controller on one midi channel.    Unfortunately, the same controller on a different midi channel will still affect your work, but it will be invisible.   So, long story short, be sure you know the midi channel of all the controllers in a track.  
 
The CAL script (save in your Cakewalk Content\Sonar Platinum\CAL Scripts directory): 
------------------
;; set_channel.cal  Set Midi Channel for selected events.
;;

(do
    (include "need20.cal")    ; Require version 2.0 or higher of CAL

    (int new_chan 1)    ; New Midi Channel number
    (int i 0)        ; index for moving through

    (getInt new_chan "New Midi Channel: " 1 16 )
        (-= new_chan 1)

    (forEachEvent
        (do
                (= Event.Chan new_chan )                              
                (++ i) ; this counts
        )
    )
       
    (pause "Adjusted " i " channel events!" )
)

Thanks William! The redundant midi event ch number can on occasion be an unnecessary confusion when working in event lists as well.

Update on this thread: I decided to correct all the channel assignments in Event List (in Reaper) as I near completion of piece I'm working on. I did this because I thought I was hearing the clarinets on the right, when they should be on the left. To my surprise,  correcting the  channels fixed this. Furthermore, the piece sounds distinctly different now. I will need to correct volumes, as pans have obviously changed. In other words, since not bothering about "correct" channels anymore, I've been mixing wrong. Now, with "correct" channels reestablished, I will need to remix everything.

So, it looks like it does matter which channel things are on. To refresh, I'm talking here about when you copy/cut paste material from one inst. to another. Reaper gets the channels wrong, and you have to correct it manually. This thread came to the conclusion that it doesn't matter, and that is the assumption I've been operating under. However, I now have empirical evidence (the best kind) that IT DOES matter. 
 
I guy on the East West forum said that it doesn't matter, except when the track inst. is sent to "all channels" (in Reaper, this is called "source." I have two choices when setting up an inst. I can send it to Source or Channel 1. I usually send it to Source. This could be what is causing the problem. Maybe I should send everything to channel 1?