It’s my belief that you should SBR Russia with at the very least the Japanese bomber, if not also the German bomber.
On mathematical average they will deal 15.5 IPCs before going down, which is about 5 infantry’s worth. After those first few battles in which you do absolutely need your bomber to do things, it’s unlikely that your bomber will cause that much IPC damage. Think, in a normal battle, in order for a bomber to kill 5 infantry on average it has to survive about eight rounds of attacks. Even the biggest capital battles don’t last more than maybe 5-6 rounds. It’s very unlikely that you will get 8 rounds of true bomber usage (your fighters should be more than enough for the small engagements prior to capital assault), and 8 rounds is simply to break even.
As for the other bombers, there is more gray when thinking about them. Germany might need every bit of strafing power available, and UK/US might need every bit of dice to overpower W. Europe, so you might not want to risk them doing SBR. Generally I would still be in favor of them SBRing, but there is a little more to think about.