HC-06 pairing to arduino
Which you use depends on two factors: the HC-06 carrier board you have, and which Arduino you are using.

If your carrier board has (and most do) a 3.3V regulator then you power the board from 5V.

If your Arduino is a 5V Arduino (such as an Uno) then you need a voltage divider on the (arduino)TX->(hc)RX pins otherwise you risk damaging the bluetooth module.