Lisa,
You will need to modify the Print.html file found in the
[Install Dir]server\apps\[App Number]\widgets\Print\templates
folder.
The line you need to change is line 100
<input type="checkbox" data-dojo-type="dijit/form/CheckBox" data-dojo-props="name:'legend',value:true,checked:true" />
Change data-dojo-props="name:'legend',value:true,checked:true" to checked:false.