When they are small. But for example in the runup to the single currency both Germany and France were in violation of the deficit rules and both simply refused to comply.
I don't really understand why this comment would be voted down as it is strictly factual. Germany and France did violate the Maastricht Convergence Requirements -- this is not a matter of opinion. They did not pay fines required by the Stability and Growth Pact. The question was about national governments' obligation to pay fines.
You can think what you like about the Euro, its implementation, the Maastricht treat in general. But let's not downvote actual data.
But the important question is, do the countries pay their fines?