I would lock it. Locking the root htaccess file is primarily to protect it from being modified by other things that you have installed on your website (plugins, themes) and not so much a security measure since bypassing/overriding file/folder permissions is a very simple thing to do. If you lock your root htaccess file and your website crashes then your Hosst server does not allow the root htaccess file to be locked and you will need to manually delete it using FTP, log back into your site and set the F-Lock option to unlock.
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