Do I not need a default deny firewall rule to block WAN traffic?

I have much experience with other firewalls, specifically Sonicwall appliances.  With those devices, I will typically have a default deny rule like this:

Is this not necessary in a UTM 9?  Everything seems to work fine with only a NAT rule, it just seemed odd to me.  Is all traffic blocked unless otherwise configured, I.E. it wouldn't know what to do with that traffic?

Thanks!