It is going to depend on a lot of variables including:
--your relationship with the boss
--the attitude of the clients toward you
--the justification or lack of justification for the rude behavior
--whether addressing the issue will lead to improvement or make it worse.
As a rule, I tried to handle things privatedly with past supervisors, who I thought were rude in the past. If something incorrect was stated in front of clients, I would try to politely correct it.
Rude behavior is never necessary, but it is in some people's nature. In today's economy, you have to balance your feelings with the need for the job and also balance if it was a isolated incident that was prompted by other events that really have nothing to do with the business, or is the rude behavior is a comment event.
At no point should you get into a confrontation with the boss in front of clients. Some of those clients may recognized the rude behavior and recognize your restraint and offer you a job. Being positive, even when faced with mounting negatives is a good idea.