For what it's worth, if you have a FS instance exposed to the world you're almost certainly better off with something in front of it.
Even if you lock down your server in such a way that no unauthorized calls can be made, the traffic is still going to make it to your freeswitch instance. Generating enough bogus traffic could materially impact the quality of active calls by wasting CPU cycles, disk I/O from logs, and memory from the connections.
Whether the "something" you put in front of it is a firewall with appropriate restrictions, a SIP proxy of some kind, or something else, is up to you (depending on the amount of introspection to the request you need). But I wouldn't recommend leaving it open.
Best,
Colin