Drag would probably force the grid fins closed. Still, the fins are controlled, so they wouldn't aimlessly flap around even if they accidentally deployed. They could in principle be aligned to cause the least drag---the same way they are aligned for steering on descent.
A bigger concern might be the perturbation to the attitude of the rocket, which could pitch, yaw, or roll if the grid fins deployed asymmetrically. But you could always gimbal the engines (or, rather, the control system could) to counter the perturbation (as is done continuously throughout flight, anyway, only difference being the source of the perturbation).
At any rate, grid fins are inefficient at low speeds---they're called hypersonic for a reason---so the drag or attitude perturbation might not be severe even if a) drag didn't force the fins shut on ascent and b) you somehow couldn't gimbal the thrust-vectoring engines to correct for the attitude perturbation.