Today we had a lot of trouble. We tried to edit a workflow using SharePoint Designer. Out user has full access to the web site, where the workflow lives. But saving the workflow was not possible. We got an "Access Denied", when the Workflow Designer tries to save the workflow. To give the user more rights, the only thing we could do is, to put him into the site collection administrators group. After that was done, our user was able to save workflows from SharePoint Designer.
It’s good, that we found a solution, but are the permissions of the site collection administrator really needed?