When to re-set a brick in a wall?
That is a new one to me. A picture of the crack would be good, but a crack that is only as long as the brick does seem to be a concern at all. If the crack was much longer, (4' and longer) with a bigger gap in the middle and tapers to nothing at the ends, that is room for concern there.

If there are only minor cracks here and there, the tuck-pointing may look worse than the cracks themselves. 

If your picture tells a different story, I will update the answer, or somebody else may chime in....