There are several examples from TOS ("Operation Annihilate!", "Tomorrow is Yesterday") and TFS ("The Voyage Home") where warp is dramatically slowed down as one approaches a large body such as a star or a planet. In TOS/TFS at least, by the time a high warp ship actually hits the surface of a planet it'd be traveling more like high supersonic speeds.
As far as TNG and it's production goes, it could go either way as their warp rules appear to be different.