We used witness cones in various locations, in a number of firings at different cones and firing speeds.
Once we figured out the little quirks in how our computer controlled kiln worked, we only use witness cones to diagnose a problem.
People using manual-kilns use them as their primary indicator of when the firing is complete. Others use them in every firing hoping to spot any problems early.
My wife just purchased a dual media electric kiln. We did our first (correct me if I use wrong terms) firing last night with 03, 04 & 05 test cones.
Do these cones need to be used for every firing?