Bringing up the firewall on the "template" VM, I noticed that I was getting more ntp traffic than I expected. I discovered that in my ignorance, I had set my local ntp server to broadcast, which I don't need. I commented the broadcast line, and everything's still working.

I also found a good post on ntp that answered one of my long-time questions: What should I look at to see if the ntp client was actually working. Do ntpq -p. On the resulting listing, "the delay and offset values should be non-zero and the jitter value should be under 100." (The post is Red Hat based, but the information specifically about ntp is distro-agnostic.)