When you refactor during a change, you make the change bigger.

When you refactor before the change, you make the change smaller.

- in a talk by Jason Swett

@jessitron re-factoring before change — helps to make it readable for yourself and might be more readable for newcomers.
Re-factoring after — helps to get review on change faster from those who are familiar with current codebase. And it also removes risk from change being blocked by re-factoring and/or need to fix/change re-factoring it is based on.