Paul, I usually practice with the same grain field tip as my broad head that way there isn't much difference in my shooting capabilities. I don't pre-sharpen my heads either. They are plenty sharp out of the package. I've got a scar on my right index finger that's proof in the pudding sorta speaking.
Anyway, I practice with the field tips and that way when I shoot a few of the broad heads, I don't tear up my targets as bad. I'm pretty consistent doing it this way too. Not a whole lot of change in my groupings at 20 yards.