A little late to the commenting party, but just want to point out that with the way lemmings are depicted to come out of the entrance trapdoor, a common interpretation is that they have already started falling from somewhere "outside" in whatever world exists beyond the opening of the entrance. So I think it's at least reasonable for one to argue that we don't actually know how many pixels of falling may have already occurred on the first frame the lemming is drawn into the level. Then given the concern of breaking some existing levels, maybe it's best to leave current behavior alone like many suggested.
If there is concern about splat ruler, I think right now you already have a bigger issue in that even in clear physics mode, there is no highlighting of the exact pixel where lemming first appears, so there is no clear point to measure against with a splat ruler anyway. Ideally we shouldn't have to have yet another splat ruler for the entrance case, but I guess since we already have one for climbers hitting the ceiling and falling, maybe it's that big a deal ultimately to have more rulers.