Create the balloon as desired and save as a favorite.
To expand on what Kevin Chandler said (at least I think this is what he meant), you can save it as a Style. Keep in mind that Styles are only saved in the active document, so if this is something you'll need for future drawings I'd suggest taking the following steps:
1. Open a new drawing with your drawing template.
2. Insert a drawing view and attach a balloon.
3. When the balloon is the way you want it, click on it to highlight it and bring up it's PropertyManager, and save it as a Style by clicking on the icon indicated below.
4. Delete the drawing view (of course the balloon will be deleted with it).
5. Save the drawing template.
Now this Balloon Style will be available to select from the drop-down in the future for new drawings started with this template.