NOT A TEACHER
Whether you use the "s" in the third person depends on how directive you want to sound. That is, if you use "s" for the third person, the statement is more directive, while if you use the infinitive form, then you sound less directive. Sounding less directive is what you want in most of the cases when using subjunctive. It emphasizes that your opinion, suggestion, demand etc. is subjective (yours only).