I've been messing with some proto shields lately and it irks me that I can't see the RX/TX leds on the main board. I've been searching and found some folks just place led's on the RX/TX pins to ground. I tried this and noticed that the TX led glows dimly all the time and the RX led glows brightly. I tested uploading/running a couple of sketches and it works fine plus they blink.
Seems like there would be more info on such a simple thing but I can't find much.
I'm using 1K resistors and tested with cathode to resistor to ground and anode to pin. I saw something about the method you describe but was a little freaked about connecting voltage to the pin.
A 1k on the red and a 470 on the green looks good.
I guess with the anode and cathodes changed from my original setup it just changed from monitoring when the pins were high to monitoring when they are low?