We had this situation occur in a restaurant I worked at a few years ago. The answer is yes. Definitely. And if they're illegal, then they could be deported or imprisoned for using false documents to secure employment. I've seen it happen. Keep in mind that once you make you're report, the only way the government is going to check it out is if you have proof. That means you're going to have to name names.
Not only that, but the subsequent audit will also get anyone else that was paid under the table in trouble, back about five years. And your name may be attached to the investigation, which could blackball you from working in the same field nearby.
Chances are, if they were paying under the table, there are other ways they're cutting corners. Find out what those are and let the authorities know.
I recently thought about this, but I didn't want to get the one person I actually got along with in the restaurant deported.