It's up to you how you would like to run your site.
If you will decide to run your site completely on HTTPS then for both Site URL and Secure Site URL in System Setting you will need to specify your site url starting with https://
Also you will need to add redirect to HTTPS for all your pages.
In case of using .htaccess it will looks like:
RewriteCond %{SERVER_PORT} ^80$
RewriteCond %{HTTP_HOST} your-site-domain.com$
RewriteRule ^(/)?(.*)
https://www.your-site-domain.com/$2 [R=301,L,QSA]