Here's something that's handier than you'd think.
When you're doing an assembly (say for a feature) and you want the image but not the audio or vice-versa rather than reach for the mouse to unselect a target track (ie...set it to nothing) try this:
For no video target track hit F6 + 0 (zero) or hit SHIFT + F6. Either way works.
For no audio 1 target track hit F7 + 0 (zero) or hit SHIFT + F7. Again, either way works.
For no audio 2 target track hit F8 + 0 (zero) or hit SHIFT + F8. Both function identically.
I find Shift + the f-key to be more convenient for me but see which works best for you. Some people like to keep track of track numbers in their head and Track 0 (zero) is "no target" in their way of thinking which is a great way of thinking about it.
The upshot of this is that since there are two commands that have the same function you can remap Shift+ the F-key to something else if you want.