They are usually done diagonal to the hill around here.  It is easiest to run a tile straight up the hill, but it also the least effective method of drainage.  Ideally I would run them with the contour, but it is harder to keep grade.  Here's an ariel picture of some of the tile we put in last year.  It is difficult to tell the ground slope from this angle, so the picture is just for your enjoyment :)

(donetile.jpg)

donetile.jpg (44KB - 199 downloads)