My usual development process is to build the site locally, then upload on a staging subdomain for client reviews, going back and fourth between the two environments for updates, before deploying it on its live environment.
From what I understand you can use the one licence key on multiple environments but you can only then login to the Admin area from one of them? Which would cause me issue when I have the staging site up but I'm working between that and the dev version?
Alternatively I can use Craft for 'Free' using a appropriate .dev or staging URL (eg. http://staging-subdomain.mysite.com) and then make sure the paid for license is in place on the live version. Is that correct?