adding LED's to Rx/Tx lines of 328 TFQP SMD to show uploading action

[BillO]

You could still connect them in a common anode fashion as the FTDI chip will likely sink as much current as it will source.  They will then only light when the data lines go low (as in when data is being transmitted/received) and produce the desired effect of letting you know communication is occurring.

[SouthernAtHeart]

But then when I unplug the FTDI cable, the pins will be low, and the LED's will be on all the time won't they?

[BillO]

No, the leakage current of the AVR inputs will not be enough to light the LEDs.

[cjdelphi]

Would a cheap npn transistor (10k on base) not do the job equally as well?

unless there's some kind of propagation delay or even such a tiny
amount of current on the base too much for the high/low signals?