You will likely receive varying opinions based on varying experiences.
In my opinion, Disney does not knowingly make people wait for tables at any restaurant....unless there is cocktail bar where you can spend money before dining.....
I am known for being 'notoriously early' for just about any appointment, and Disney ADR's are no exception.
More often than not, I check-in early (which typically means no more than 15-minutes early, as per Disney policy). Sometimes I get seated before my scheduled ADR; sometimes I wait.
In other words, I don't believe there is only one answer, just varying experiences.
But if you want advice about the virtue of an early check-in, I'm here to encourage that notion at Sci-Fi or anywhere else.