Finally found out why our deploys sometimes showed old CSS! It wasn't stagecoach's fault at all, it was the minifier. Our minifier was looking at LESS only if the CSS did not exist. So if there was an old, stale compiled CSS version of a LESS file present (as the result of deploying from a dev workstation other than that where it was tested), it would use that stale code. Now the minifier always looks for LESS first and always uses that rather than trusting what the non-minified dev environment might or might not have done already.