I see no problem at all practicing with field points if they hit the same place as your broadheads. You are minimizing wear and tear on your target.

I test my broadheads about once every week or so to make sure everything is still lined up properly but most of my shooting is done with fieldpoints.

As i get closer to the season I start adding more and more clothing to my practice session to the point that for the last 2 weeks or so i am shooting with headnet, gloves, jacket (thank goodness my basement is cool), binos and a tube call around my neck