I design a PCB that one of its components is a 3.3v Pro Mini. Since I want it versatile regarding powering the PCB (3.3v or 5.0v) I want to know if it is a good practice to supply 3.3v to Pro Mini's RAW pin( rather than to the VCC Pin ).
It is not ideal, no. The RAW pin first goes into a 3.3V voltage regulator whose dropout voltage is up to 200mV (worst case). That means that you would get around 3.1V out, and it wouldn't be properly regulated.
Oh, it would work, but the power won't be stable, which means that any analogue readings that you make that don't directly use the (shakily) regulated "3.3V" (now 3.something else V) in the generation of the analog signal (such as a simple potentiometer) will be inaccurate.
Also output logic voltages will be lower than expected which may have an effect if communicating with something whose upper input threshold is at or near the 3.3V mark (such as many 5V devices).