IDPA Rulebook Page 19 Section 2 Stock Service Pistol Item H:Began Hammer down for selective DA/SA pistols. I shoot a 92FS IDPA and carry a 96FS on duty and CCW. I carry from the start position hammer down with a round chambered, decocker/safety in the up/off position.
The Beretta has a firing pin safety inclusive in it's design that precludes the firing of the weapon without the full travel of the trigger to the rear, much like a Glock/S&W M&P which is a striker fired weapon.
The Beretta at the start position with a round chambered and hammer down is no different than my fellow competitors carrying revolvers either. I have never been required to carry with the decocker/safety in the down/on position. Just check the rule book and your Safety Officer. I wouldn't dream of carrying the weapon in the decocker/safety on position in normal operation as a reserve deupty.
I have on occasion reached to the weapon and engaged the decocker/safety in a very restrictive room situation where there were many young people present as a safety precaution should on try to grab the weapon. In this case I run my thumb under the safety while drawing the weapon and disengage the decocker safety as the first operation of drawing. My normal thumb position does this naturally at any rate. Enjoy the shoot and the weapon, you should do well with this choice and have a great time.