Email marketing notoriously falls last-minute on most clients of mine -- the client wants it to look perfect everywhere and to be sent yesterday, with their superior watching over their shoulder until their email is sent. As such it's commonly a sales pitch to get one to agree to split test any sort of content changes.
Just last week, a client wanted all of the text to appear in the exact font provided across all email clients, requiring flat images to be used instead of live text. After a discussion on the benefits of live text vs. images and fonts available in various clients (and helpful Litmus checklist results to help explain the differences), we finally agreed on split testing the two versions to see how each one performed. As expected the live text version performed better, even though the preferred fonts weren't supported across the board -- and we'll use this information with future sends for this client!