A thought about this: Years ago I would have agreed, but now I'd strongly disagree, because of the noise it can add to diffs. When you're triaging a bug, or trying to figure out what effect someone's change had, you frequently go back through old commits, and look at the log message to see what the change was supposed to do. Perhaps a bug or side effect was introduced.

If the diff is full of additional formatting/stylistic changes, it can make it a lot harder to cut through the noise and get down to the heart of the change.

I'm not saying you should never clean up old code/files, however when I see that needs to be done, I try to do it in a separate commit that I make after the "real" change. The log message for that change is something like "Formatting", or "Cleanup, no functional changes".