i've made and shot primative arrows at stumps and bunnies, but now i've got some flint heads to try on deer.
when shooting stone heads:
how straight is straight to get good flight?
how do you practice with stone heads?

thanks guys

shoot them into bales of straw into the sides of the bale or use a pile of sand

I use a big styrofoam block for flint heads.  Sand will dull them very quickly, which is okay if you're going to resharpen them or they're dedicated to practice.

I shoot them into a rubberized foam block , generally only once or twice to be sure they fly well.
 If the arrows you are shooting at stumps and bunnies fly well, add the stone points and see how they fly. They don't have to be as straight as most folks think. If the point and nock line up and they spin true they should fly well.