For me it works nicely at all times..
There are the "new" kind of guard that spawns though on some maps (Framing Frame Day 1/3 for example). Killing him will -always- raise the alarm, and on FF Day 3, he is identical to all the other guards.
I've seen this guard once on the new Heist as well.. Killed him as the second kill, and the alarm got rung seconds after he hit the floor. Even with F7 active.
Also be aware that while F7 works normally, on the Armored Truck heists, the alarm seems to be hard-coded to be active as soon as the map starts, so they will always spawn cops. No way around that as far as I'm aware.