I have a good friend who makes a portion of his income in aviation photography. He has stuff on display at several museums including the Smithsonian Air & Space Museum, the museum at Oshkosh and the Wright Brothers Memorial Museum. I'm always stunned by his images, they are truly great pieces of work technically and artistically. He has a variety of prop and rotor images both with and without motion blur in the blades.
I think it depends on the context. For instance, military choppers look more menacing and lethal if the blades are frozen or showing multiples from a harmonic effect. Old fixed wing aircraft usually look better with blurred props... certainly aerobatic planes look better that way. Most of his jet images, (he has killer access to military aircraft) are frozen. He has this one series that is just too cool with WWII warbirds paired with modern counterparts—a P51 Mustang flying wing on an F15 Eagle for instance... and now that I think about it, I don't recall if the props were frozen... I'll get back to you on that.
I love aircraft but have done no aviation photography. I always wanted to get a slow exposure from the cockpit doing spins but I never got around to it. :P
Chip