You can't change the default settings so that the IPTC titles and captions are used as soon as JuiceboxBuilder-Pro is opened but you can save the settings in a preset file.
Select 'Images -> Titles -> Use IPTC Title' and 'Images -> Captions -> Use IPTC Caption' from the drop-down menu at the top.
Now go to 'Presets -> Save Preset...'.
As long as you save the preset file in the default location, you will be able to select it quickly from the 'Load Preset' list on the 'Customize Tab' (or via 'Presets -> Load Preset...'). (Maybe not perfect but it halves the number of clicks required to load IPTC titles and captions: 2 clicks now instead of 4.)
Please see the JuiceboxBuilder-Pro User Guide for more information on Presets.