What setting do I change to make lock proportions be on by default?

I'm not sure there is a permanent quick fix, but, I you could make a new 'default template' where you make that change, then save it. There are specific instructions on the forums here (do a search on default templates) on how and where to save to. It's pretty easy (even I can do it, lol).
I've made many changes to the default, examples, changed the size, removed the shadow on the border, I now have 3 layers vs one, etc. Just about anything you do to the template and save, will stay, so, if you click on the lock, it will be your new default.