In truth the rules for calling a criminal tax attorney are a lot like the rules for calling a regular criminal lawyer. The sooner the better. But what if you don't even know you're in trouble? What then?
Let's look at an example to help us decide what to do. Let's say you are Joe Schmoe. A normal guy, working a normal job, paying normal taxes. For whatever reason you are audited by the IRS and they call you to let you know you are being audited. They tell you the have a couple of basic questions for you so they can get a little bit of background information on you.
Stop. This, believe it or not, is the time when you should keep your mouth shut and go talk to a criminal tax lawyer. I know there's no indication that you're in trouble. I know they just want some background information. But guess what? Anything you tell them can be used against you later to try to put you in jail. And the crazy thing about it is, while you are answering the questions, you don't know if you are implicating yourself in something big.
Hiring a criminal tax attorney is not like hiring a truck accident attorney. You don't want to wait until the accident has happened (or you've been arrested) to be looking around for someone. You want to be out in front of something like this. And a criminal defense lawyer can help you do that.
And you're right in thinking that the feds might be a little bit miffed that you lawyered up. They may think you have something to hide. Don't worry. Soon they will see that you don't have anything to hide, that you are simply exercising your rights, and that you understand that you have no obligation to make their job easier (just like they don't make your job easier with answers to your tax questions!).
In the end, the general rule for talking to a criminal attorney if you have tax issues is as soon as possible. Don't wait. Get in and talk to someone to find out what you should do. When the whole matter is resolved, you'll be glad you did.