Uncertainty and Improvement: Harnessing the Desire to Improve

"I'm implementing a user story and I see a way to improve/refactor the existing code while I am at it. Should I do that?" "I've completed implementing a user story and want to clean up the code I've written. Should I do that?" "I have an idea for a better way to solve this problem. Should I implement it?" "This piece of legacy code is complex and painful. I want to rewrite it. Should I do...