It depends on the toilet, really. Some have jets that force the water to swirl in one way or the other. In my toilet (I can't believe I just went up to flush it for this) water spins counter-clockwise, but that's mostly because there's 1 jet of water on the right side pointing down and to the wall, 1 on the left side pointing down and to where I'm standing. So there's no Coriolis effect deciding on it; the toilet manufacturers did it. However, what if there weren't those jets? As it said above, the Coriolis effect is just plain too small to do anything. It has a huge effect on the weather as this
link shows (Scroll down to "what causes the wind to blow", about halfway down). It changes the winds a lot. But, the toilets? If the manufacturer put those little jets, that will determine. Otherwise, it's random, really; there's so many variables that determine it. Probably, given 1 million toilets without jets in the northen hemisphere, 500,010 will go one way and 499,990 will go the other. There is an effect, it's there, but it's really freaking small and in no way is it a law.
...I need a hobby.