I have a question about the voltage divider circuit you're suggesting.

You say that it isn't needed on the arduino RX pin, only on its TX pin. Is that because the voltage is truly directional for RX and TX like that (i.e., the voltage is always driven by the sender on that pin)?

In other words, tx from a 5v device is always 0v for either not transmitting or for bit 0, and then always +5v for bits? And then RX is always an input and never outputs any voltage?