In the phrase "none of us is promised tomorrow", should it be is or are?
There is a subtle distinction in focus, in that the is form focuses upon each member of "us" separately, while the are form covers all of "us" at the same time.
Now, this pretty much amounts to the same thing, though it could make one fit into a piece of writing or a speech better than another.
If you want to turn the focus from the general to the specific (particularly if addressing an individual and wanting to focus on the person addressed, or if you want to turn from the general to your own position), you may therefore be better off using is.
If you want to stay focused on the generality of the statement, you may therefore be better off using are.
The plain meaning, rather than rhetorical focus, is the same either way.