The best answer is, maybe.
It is definitely better to have a positive air pressure in your case (more air blowing in than out). It will keep your case much cleaner. With only an exhaust fan you have a negative air pressure going on which means that air (and dust) is being sucked in from all the crevasses around the case.
As far as your question of, "Does this reduce the temperature...," that depends on the current temperature inside your case when stressing your components. If your case is getting decent airflow then the change in temps will be minimal but there's really no downside so I'd say go for it.