Also be aware that your Tx pin may actually power up the FTDI chip depending on how it is constructed.
The other thing to do is make sure your processor's Tx pin is tristate when the USB is not plugged in, if you can know that.
both RX & TX go high when USB is unplugged?
You only get 500mA from USB, I don't if that's a concern.
I see, the datasheet shows no pull up on the reset line to 5V, and the pin descriptor says it is not required - leave unconnected or pulled up to 5V. However - I shall trust your experience.