The timing is dependant on the X position of the sprite, and I'd think also the X position of the collision (give or take a few pixels - a single pixel sprite #0 is known to be unreliable).
It is? I thought it just appeared unreliable at one point because it doesn't work on x=255.
That's what I thought as well. To my knowledge there are several commercial games which use 1-pixel wide sprite-0 hit reliably (Qix, iirc, possibly Battletoads, I think some Dizzy game).
Blargg's test ROMs might also test this.
I can't verify it, but I'm pretty sure I remember hearing that from a couple different people. My sprite #0 was always a horizontal bar, so I never would've ran into it, if true.
Battletoads hits a big sprite over a 1-pixel target, and always hits properly.
So does most games by Rare.