and I wouldn't recommend that any pump be dead headed for any extended period of time, I have come across a Taco 007 that had been dead headed for 2-1/2 winters, and it was just fine.
It's the BIG pumps that you have to worry about. When dead headed, you are pouring a SIGNIFICANT amount of energy into a small amount of water and super heating it. When the pressure is dropped, the water is no longer water, but becomes steam and occupies 1700 times more space than it used to when it was water.
I once saw the power head from a Grundfos circulator that had been blown through two sheet rock walls and embedded in a cinder block wall. It can happen, but it takes time and when talking about small circulators is not as critical.
Better safe than sorry just the same.
I have also seen, and provided the opportunity for :)deadheading small wet rotor circs.
They can take quite a beating with the discharge valve turned off! If both inlet and outlet iso's are turned off, you generally get minutes, not days!