If the heads are on the bench you might take one more thing into consideration........When you're tapping a hole you have to make sure that the item you're tapping is secure. If the heads are on a smooth (slippery) surface they might tend to move (rotate, shift) when you're applying torque with the tap. If the heads move at all it can cause the you to put uncontrolled force on the tap causing the tap to break. But if the heads are bolted or clamped down securely (or in a large vice) this can't happen. I know that I've tapped items that shifted and caused me to break off a tap. It was one of the smaller taps but still, so be careful and make sure that the heads can not move during the tapping process.
Making the heads secure will also give you a much better feel as the tap cuts the threads. If you noticed in the short video I posted, the guy has the small part in a vice!! The guy in the video also used "Tap Magic", I also use Tap magic when I'm tapping threads. Tap magic allows the tap to cut threads with a much smoother action which decreases the chances of breaking the tap. Once you tap a hole using Tap Magic you'll never want to be without it.
As JBurch wrote, cast iron has properties that can be considered "self lubricating" when it comes to tapping or machining.