At my current skill level, especially since I haven't been riding as frequently, I almost always start from the very bottom or towards the middle/last third of the hill. It definitely depends on the length and grade though.
If it's something like the shallow, rolling hills I get on my commute, I wait until the top to give myself an extra push on the way down. Anything much steeper and I start closer to the bottom because I have a hard time building power up halfway through a climb once my legs start to get tired.