I can offer a couple of recommendations:
1) Enable settings inheritance and configure your registered computers to inherit settings from their parent folder or from a global settings object. And then on either folder or global level you can configure default screen stretching mode for your VNC connections (under Remote Clients \ VNC \ Settings \ Display).
2) If you need to reset settings on existing connections, you can do it via Tools \ Options \ Registered Computers \ Default Registered Computers Settings
First you need to configure desired settings by clicking on the Edit Default Settings... button and then reset settings by clicking on the Apply to All Computers... button.
Kindest Regards,SmartCode Solutions Support