First, this is a workbook by workbook setting.

You can use:
File|SaveAs|Tools|General Options|check the always create backup box
For any existing workbook file.

For new files, you can create a template that has this setting toggled.
Start a new workbook
Make as many changes as you want (including always create backup)
And save this file as a template file named book.xlt in your XLStart folder.

Excel will use that for new workbooks when you start excel and when you click on
the New Icon on the Standard toolbar.