RX and TX lines at TTL signals** between the μC and the line driver IC*** is a good place to connect the LED indicators. There's no good reason to connect the LEDs to RS-232 lines. RS-485 lines are bidirectional, so the LEDs alone wouldn't tell if the data is TX or RX on RS485.
** 0 to 3.3V serial
*** RS-232 or RS-485 line driver, and such
Back to TTL signals. You can connect the LED between signals and GND **, or between signal and Vcc ***. Either would work as long as the LED current is low enough (limited by a series resistor). LEDs add some capacitance, which might be a problem at high speeds above, say, 1M bits per second.
** source in the O.P. terminology