Switching from 12v to 9V

If I'm going to use 12v and 9v to supply an arduino. Do you think is secure to switch does two voltages during operation using ORing diodes?
Should I place a voltage regulator or something for protection?

ORing diodes is sufficient.
Are you expecting the Arduino to source a lot of 5V current to a shield, LEDs, anything?
If so, an external regular to bring the 9/12 down to 7.5V before feeding into the barrel jack connector, or 6V to feed into Vin, would be a good idea.

CrossRoads:
ORing diodes is sufficient.
Are you expecting the Arduino to source a lot of 5V current to a shield, LEDs, anything?
If so, an external regular to bring the 9/12 down to 7.5V before feeding into the barrel jack connector, or 6V to feed into Vin, would be a good idea.

how can I brig down both bot voltages?

A this moment I'm planning to interface a temp sensor(LM35) and a RN42 bluetooth module but maybe I'll add a gsm/gps + sd card, but I'm not sure if I can interface everything at the same time.

OR thru the diodes into the regulator.
Check your bluetooth and its current draw - many are 3.3V devices.

OK thanks for your taking your time to help with my doubts. 8)