The piston does try to slide down the slope so it is crooked back towards the chamber. To stop it I have that oval shaped washer I made out of PVC on the face of the piston. You can see in the down the barrel view that it is resting against the bottom of the sealing port. I made one a bit longer on the top than in the picture after I realized it was sliding down.
If you mean lined up as far as the piston turned at the right angle, then as long as it isn't significantly rotated from where it should be, the downward force of filling the gun will naturally rotate the piston into the proper orientation when the sealing face contacts the port.
Bone: I ordered the rubber from http://www.mcmaster.com
It was pretty difficult to get this particular valve to seal, but it was also complicated by several other issues I had with the valve.
While it isn't a lot different from a standard valve I certainly wouldn't recommend it for a first.
Thanks for the compliments.