FX BIN CPU usage question

Unfortunately, the code within most plugins continues to run during silent periods, even if it's just processing zeroes. It just doesn't have any way of knowing whether the "silence" is truly silence, and strictly speaking it often isn't. The only way to guarantee zero CPU usage is to render those tracks.
 
The good news is that freezing and un-freezing tracks is quick and painless. Even if you only freeze those tracks that you're pretty sure won't need additional tweaks, that can make the difference between your CPU being able to keep up or not. If you later realize you do need to make effects changes, you can get the original track back in the blink of an eye.
 
At the risk of stating the obvious, you only need to take CPU-conserving actions if you're getting dropouts. Your system can either keep up or it can't - there is no benefit to reducing CPU usage if data is already making it out to your interface fast enough, even if the meter shows a frighteningly-high amount of CPU usage. And this is assuming you've already set your buffers to their maximum, which you should be doing as a matter of course. Latency is a non-issue when mixing stems.