I wholeheartedly agree. Most of a Candidate's resume worth looking at, happens before their Senior year.
Do not take that to mean that you should coast in 12th grade, however. It's just seems that it is so infrequent that the folks in Admissions even ask for Senior year grades, that it appears that they don't really put much weight on what you do after those first 6 semesters - unless you make it to Salutatorian or Valedictorian - I suppose. That would be what we call "a good thing".
On the other hand - if you really do something stupid - like tank all your grades or end up in jail, that would be what we call "a bad thing".