After about three weeks working with the trial version (plan to purchase license this week), regarding the auto-save feature I would suggest the following (item number one has a bit more than just auto-save):
1) Create a Preferences (⌘,) option under the Balsamics Mockups 3 (as with majority of Mac applications) and mechanism to turn auto-save on/off, so as not to require for each Project created (move other options here, as well ... font name, font size, default container sizes, default text within containers, and many other application wide settings);
2) Create a Revert (similar to feature in all Adobe Creative Suite apps, or "Revert to" if versioning is used (similar to Apple suite of office apps--Pages, Numbers, Keynote);
3) Create a recovery file where auto-save writes changes, and this file can be discarded for "Revert" or applied if "Save" is selected.
As I stated above, I am a Balsamiq rookie, and I believe these features would improve the functionality of this app immensely when experimenting with changes and/or saving files (Projects).
Thank you for your time and consideration ... Mock On! 😎