Earlier quoted context omitted.
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?
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".