How to effectively use the saw to open deposit boxes and doors?

Click, do not hold. If you hold you could accidentally use the blade once. Two clicks for deposits, about five for doors, and two to open ATMs.

Just gently tap fire 2 times on deposit boxes and exteriors of ATMs, 5 times on metal doors. Don't waste blades on wooden doors, just shoot the doorknob in, and carry as much ammo as you can, including both the ace of the original skill (allows for one extra blade).

Use the skill above granting 20 percent harder blades (I don't know if the 25 percent extra ammo skill affects saw blades, however). It's really only useful in the bank, personally I reskilled myself in technician. Shaped charges are better for getting through doors and you can carry a gun that actually kills enemies at distance.

I was playing with some players recently who were talking about the "Beserker" skill. They had stated that by letting their health drop to a quarter, it meant their saw was 3 times as effective. I'll respec eventually and test it. Consider it though. Using it as standard, I found short bursts, and relax.