I don't think there are preferences for those but you might be able to create a preset for that: Create empty graph builder, go to legend settings from red triangle menu, modify them as you would like them to be and then save your preset. And when you wish to use this, you can quite easily load it from the preset menu.
Also if you wish to be able to modify those from preferences, you can make a wish list item to JMP Wish List
-Jarmo