On any type of stairs that are facing West or East, trying to connect a string to another one in front of it (Based on East and West) will not work, and the string will just rather stay in the direction it is in.

As seen first screenshot, the string on the stair will not attach to the one on the block, but as seen in the second screenshot, it will attach across 2 stair blocks. And although the second shot looks like they are attached, they are not. I added a tripwire hook to it, and it did not create a redstone signal by stepping on either of the tripwire pieces.

I've tested this in all 4 cardinal directions and with every stair material. I've gotten the same results with the East/West directions. I've gotten slightly different results with the North/South direction, though.

In the third picture, the string would seem to be attached, as they are going in the same direction, but, after attaching a tripwire hook to both sides, stepping on either tripwire piece does nothing in the ways of a redstone signal.

In the fourth picture, you get the same results as in the second picture, the tripwires are connected, but will not create a signal.

Altogether, putting a tripwire in any direction over stair-block or stair-stair will not create a redstone signal when attached to a hook on both sides. i've only tested this on Minecraft 1.8 pre-1, so I'm not sure if this effects any previous snapshots or full versions of the game.