I suppose that depends on your asessment of your risk. Here are the combinations:
1) port 3389 always open on the router, remote admin disabled, remote desktop enabled.
A) port 3389 would be visible to anyone scanning your public IP address, so an attacker would know that you're there.
B) attacker would have to guess or brute force crack a valid username & password on the windows machine or take advantage of some exploit before it was patched automatically.
C) if exploited or comprimised, it's likely that the entire computer would be accessible.
2) port 8080 (or whatever) always open on the router, but nothing else externally accessible.
A) port 8080 would be visible.....
B) attacker would have to guess or brute force crack a valid password (I don't think that username does anything on this device) or take advantage of some exploit before it was patched manually.
C) if exploited or comprimised, the entire router would be accessible which means that RD 3389, Windows File Sharing 135 & any other port could be forwared to the computer that's on & listening for connections.
So, my thoughts are that you're probably more secure by leaving RD open & closing the router admin. To help secure things just a little further, you can change the RD listening port