Hi,
Yes. If you don't, the first link generated will be the one used.
Usually, it's best to proceed differently.
Instead of doing what you describe, set the "Force Canonical on Listing" setting to "Use Canonical URL if available".
For all the products already with their correct canonical URL, it won't change anything.
For the new products that you then create, the system will generate the URL on each listing page. So you'll have all the time you want to browse your website frontend and copy/paste the URL that you want as canonical URL in your products.