Only the people on both extremes of opinions will rate a drama before it's completed. It'll probably be better reflective at that point (and a little while after when the bingers get ahold of it - since not everyone watches while airing).
But I never go off ratings anyway. It's too subjective. Even if people like the drama the exact same emotional amount, the star rating might differ. For instance, I rate things a lot harsher than some of my peers, despite 100% agreeing on our sentiments about it.
And there's plenty of hidden treasures that people pass right by because it has a low rating for some reason (not enough viewers, not the right audience, what have you).
So i don't understand this obsession about what the rating is or how "accurate"/"deserving" it is of it.