tajen advocated a CA-issued certificate along with the "require" option, which makes sense under no common scenario. If your network really is secure, you don't need TLS at all, so you can use "disable" and not bother with a cert. If your network isn't secure, then you need "verify-full" to actually have security.
actually i just wanted to point that out.
actually on aws you actually connect with ssl when you have an rds instance and using psql on a ec2 instance. however it will run with "prefer". which isn't really needed there.
If you consider the Internet a secure network you can. Note that unless you connect to IP addresses you still need to do host name resolution which typically involves the Internet and if the Internet is considered insecure, may return answers controlled by an attacker..
I'd suggest avoiding shortcuts and using certificates.