I have an easy rule. If I am using latex I do floors first. If I am using oil, I do walls first. Oil can be a bitch to clean up off of floors but really easy to clean dust/whatever off the walls. Latex is basically the opposite. You do not want to wipe/clean a latex wall for probably 2 months after painting. Also if the latex has not cured for at least 3-4 days (maybe longer) it can hold dust spores permanently.
