I would not leave it default...it may not be used often but it can be exploited for bad things. For example, somebody connecting to it with the default username/password, pointing it to a rogue deployment server, pushing down scripts to run in context of the splunk user and possibly owning the box.
On the UF's, we set a random password for the admin account and disable the management port.
Have a look at this .conf session from a couple years back:
https://conf.splunk.com/files/2016/recordings/universal-forwarder-security-dont-input-more-than-data-into-your-splunk-environment.mp4
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