I trimmed my first BC when she was a senior. Her coat had gotten wooly and she had hot spots. She really didn't spend a ton of time outside. It was mostly to let the hot spots breath and heal. Her coat never grew back right so we kept her trimmed after that.
Now I do trim pads and the Lorax feet. Also, I trim the pants on one of my dogs. But that's about it.
As for getting rid of undercoat, I try to brush frequently with an undercoat rake. And a couple times a month (more in the summer) after they've gotten wet, either in the stock tank or at a beach, I use a forced air dryer on them. The dead hair flies off! No one likes it, but they all tolerate it well enough.