I was going to raise the point on the number of bars, as well, as this may heavily play into when coasting is used versus braking to regulate the speed. Perhaps the more bars (further distance) you're using, coasting is a more acceptable option for regulation of speed. The closer you are, perhaps braking is more necessary?
I tend to use only 1 or 2 bars. My car defaults to 4 bars, and the distance on the highway is tremendous. Even at 1 bar, it appears to be at least 2 seconds behind the car in-front. At 2 - 3 bars, it encourages a lot of people to cut-in... but this is in heavier traffic.
At 2 or more bars, I've never experienced an issue with emergency braking. The car seems to respond well and can bring the car to a complete stop - and take off again when the car in-front begins to move. However, with only 1 bar, I've experienced the collision warning system kick-in when using ACC under emergency braking at higher speeds. My understanding is that ACC can only apply "some portion" of the car's braking force - and it's quite substantial from what I've seen. However, in true emergency braking situations, under very heavy braking of the car in-front, the collision warning system has blinked and beeped at me, and I've responded by applying the brakes myself. Would ACC have stopped the car regardless? I don't know - and I didn't want to find out.
After re-reading the manual on the above subject, it appears that if the collision warning kicks on, you need to manually press the brake pedal - the car may not be able to apply enough braking force alone via ACC. I've had this scenario happen a handful of times, and each time I was able to safely stop without using all of the cars braking force - no squealing tires or ABS kicking-in, etc.
So, I'd probably conclude that more bars may allow the car to regulate speed more frequently via coasting versus braking, but less bars may cause the brakes to be used more heavily to more strictly maintain the smaller distance. Obviously, I don't know this for certain, but I may experiment with it to see. I also believe 1 - 4 bars are all completely safe, but clearly the 1 bar setting may result in situations as I've explained above, where collision warning kicks on and causes you to intervene.