If you have a fairly secure computer how dangerous is it to connect to a modem directly without a router in between.

Let's say the computer is running Debian as a host and Whonix in VirtualBox, a VPN on host and has the appropriate firewall rules in place.

Would it still be important both for security and the integrity of Tor functioning properly to have a router (DD-WRT) in between the modem and the computer to act as a firewall and an added layer of protection or is it irrelevant.

This is all assuming nothing else is needed, no network sharing or VoIP or VPN hosting or anything like that, simply internet access to the computer which the modem by itself can provide.