My memory of the translators' approach, from reading a preface or some such at one point, is that they kept every pun they could, but if they couldn't manage to keep one they made sure to add another pun in somewhere else in trade.

(And yeah, Idéfix is a joke, but it doesn't have the double layer of the "dog" pun. Goscinny and Uderzo were apparently delighted by that one, in an I CAN'T BELIEVE WE DIDN'T THINK OF THAT way.)