Having nothing listening on port 80 and having the firewall block it are basically identical from a network perspective: either way, a bot sends a packet, and gets back either a response indicating the port isn't open, or else no response at all. So unless the bots know in advance that you don't have anything listening on that port, it's not going to save your firewall or your network anything to actually not have anything listening on that port.

Assuming that bots attack across all valid IP address space fairly evenly, you could reduce the impact of these attacks by having fewer IP addresses routed to you. Using nonroutable IP addresses (such as RFC 1918 addresses) might help with this. If you use private addresses on the client and server network and configure routing on both ends to ensure only your real clients can talk to your server, your firewall won't have to deal with packets sent to these private addresses, since in general they can't be sent. You could get a similar effect with a nonroutable protocol (such as NetBEUI), though it would be quite a bit more work. Both of these are most easily done through a VPN, though they can be done with physical network links as well.

Still, the network and router resources used by blocking a connection are so small, going to these great lengths won't make much of a difference on the vast majority of networks.