In most US states, the 'legal limit' as detected with a breathalyzer is just a threshold that results in automatically being charged with a DUI/DWI if you exceed it. Blowing under the threshold doesn't provide you any kind of legal protection against being charged with a DUI/DWI.

So, in short, you should never blow, as that's never going to help you in any way?

In most states, refusing a breathalyzer test can cause suspension of your license, even if you are completely sober:

https://dui.findlaw.com/dui-arrests/can-i-refuse-a-breathaly...

Sadly, as with many things in our criminal justice system, the answer ends up being "hope that you're not prosecuted by mistake, and that you have enough money for a lawyer if you are".

Thanks. I thought it might be something like that.

The main thing you're missing is that the official story of the "justice system" is a fairytale. Once you're in its sights, their literal day job is to make your life hell. They get paid to do this and go home at 5, while you pay for and live it.

Presently its damages are externalities paid for by a fucked-up reverse lottery that we're all playing, most blissfully unaware. A significant fix would be to make the state responsible for the entire cost of all defense, days spent in a cage or court, any bail fees, and emotional distress. Having to pay compensation for their "mistakes" would obviously further increase the scumbags' perverse incentive to obtain convictions/pleas. But you'd hope after they had paid out enough for persecuting frivolous cases, budgetary constraints would necessitate a bit of downsizing.

A breathalyzer is just additional evidence. All the cops have to say is the defendant appeared to be on something.