Can I leave ports open on push-in type wire connectors?
Yes.  If you choose to use push-in type wire connectors, you can insert 1 to #of_ports wires into them.

As mentioned in the comments... Some folks don't trust these push-in devices, because of the failings of their back-stab brethren. These devices are listed, and allowed for use by the National Electrical Code. But you have to keep in mind, so were the failed back-stab connectors.  So use at your own risk.