As far as I am concerned, the major spoilerific stuff is done. As for the details of the plot and so forth, I don't think the shallow and dumbed-down version they show on TV is going to affect those who read the books for the deeper stuff.

In a way, the show has already started spoiling things by omission. Since GRRM is so involved with the show, we can safely assume that a lot of the decisions made are done so with assurances of their long-term feasibility.
"Replace Jeyne Westerling with a character more calculated to pander to modern sensibilities? Sure. Her character development isn't a big deal."
"Kill Robb's wife at the Red Wedding, for extra shock and gore value? Go ahead. Jeyne doesn't do anything important in the books from here on out."

Other characters who have effectively been written out of the story by circumstances precluding their belated appearance, such as the case of Roose Bolton, or the Tullys, could also be spoilers. For instance, we can safely assume that Loras & Margaery's elder brothers are going to be fairly insignificant, since the show has made a big deal of the detail that Loras is an only son, and his father's sole heir. We can also probably write off people like Edric Storm, whose roles have been taken over by other characters.

It also makes me wonder what else they are going to cut out to fit such an abbreviated schedule. The two books since 3 have not been any simpler or more easily translated to on-screen action, IMO (in fact, I read somewhere that the writers are dreading the task of plotting out season 5, since there is no easy action-oriented point in book 5 that could serve as the equivalent of "Baelor," "Blackwater" or "Rains of Castamere" ), so if they are going to end season 7 with the same general stuff as book 7, an awful lot of supporting plotlines will have to be trimmed. And the thing is, they seem to be painting themselves into a corner by leaving even more stuff for the future. As far as season 4 goes, it very much looks like Tyrion's whole arc is going to be the back end of book 3. That's fine for Tyrion, who is absent from book 4, but that same plotline intersects with some characters who are not. What are Cersei & Jaime going to be doing while waiting for Tyrion to kill their father in episode 10? So much of what they did in book 4 is informed by that event. They are going to have to make drastic alterations, or else keep rolling this stuff further off, making it impossible to cover the book series in a mere 70 episodes.

So there is going to be a TON of stuff chopped out, and the books may not have anything to do with the show at all by the time they come out.

On the other hand, what if having the TV series in the bag influences how Martin writes that stuff, and he ends up skimping on the end and abandoning characters and plot lines that do not get serviced on the show? As far as I am concerned, that's a far worse danger than spoilers.