I can't believe someone hasn't responded to this question yet. The simple answer is - either way, but a lot depends on the geometry of the cock/frizzen and the design of the flint itself. I use the dark grey flint knapped by Tom Fuller. I start with the beveled edge up and test for spark. When the flint is worn, I will turn it upside down and get a few more strikes out of it. When the flint no longer sparks consistently - change it out.
An old trick to use when you are out in the field and find the the flint edge is too worn to spark: Leave it in the jaws and pull it back to full cock. While holding the lock back with your hand, strike the sharp (but worn) edge of the flint with something heavy and metal - like a screwdriver, back edge of knife, etc. This will give the edge a "snaggle tooth" appearance, but will usually be good for another couple of sparks.
