The conservative view would be to recommend that as a best practice. However, we are already relying on Tor Browser not to generate any unique identifiers every time we use it. So logically, it should be safe to run in a template for customization purposes. But it really needs to be stressed that customization is a dangerous exercise. For example, I found by accident that a seemingly innocuous change:
NoScript -> Options -> Notifications -> check 'Show message about blocked scripts'
will alter reported screen size when JS is enabled.
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