I've made changes to my default templates and they don't seem to "stick".
We've found that this may be due to a little corruption in the pref file, or due to the user, not quite applying the instruction to the right section of the Inspector. Note, that you can set templates per Document or per Journal.
If you are pretty sure you have done it properly, let's delete your MJ pref file. Quit MJ and navigate in a Finder window to your your user
folder's Library/Preferences folder. Look for...
...and move to the trash. Launch MacJournal and apply the template to the appropriate Journal or Document.