Used my multimeter to check this - seems that it is correct.It turns out that the RX pin on the Arduino is held at 5V even when that pin is not initialized. I suspect it is due to the fact that the Arduino is programmed via these same pins every time you flash it from Arduino IDE, and there are external (weak) pullups to keep the lines to 5V at other times. So the method described below may be risky – I suggest either add a resistor in series to the RX pin, or use a proper level converter (see my other post on how to accomplish that). And if you do try the method below, never connect the Pi before you have already flashed the program, otherwise you may end up with a damaged Pi!!!
This article and some others across the Internet, however, says that straight connection RPi Tx to Arduino Rx is ok. Who to believe? I am afraid to brick my RPi, so I want everything first well planned.
Voltage dividers are obviously unidirectional (unlike MOSFET). And if the signal goes in the opposite direction, it stays the same as on the input? That is, if I put a second voltage divider on the line RPi Tx -> Arduino Rx, it will not affect the levels, but theoretically could protect RPi from 5V on Arduino Rx pin, am I right?