While this may be the rule on paper, if you have something like "Mechanical Engineer" on your LinkedIn profile, even without the word "Professional" you'll have

While this may be the rule on paper, if you have something like "Mechanical Engineer" on your LinkedIn profile, even without the word "Professional" you'll have your engineering organization send you a scary letter asking you to remove it or face the consequences. The reason is that the "Professional" part is usually implied by the "Engineer" part.