If you're building a diorama of a troop insertion or extraction, with the helicopter on the ground, it'd probably work best with the main rotor still and the tail rotor blured... That's what people are used to seeing in photos and their mind will "fill in the blanks" regarding what's supposed to be moving and what isn't...
Dioramas with "action" taking place are best when they depict a "snap-shot" of a moment in time and you shouldn't mix animation with stills. The key is to put figures and vehicles in an "off-balance" position, one that the viewer knows would result in a guy falling down if he tried to strike the same pose standing still...
Real smoke, muzzle flashes, "flames", flickering lights, and spinning props & rotors, although kinda "cool" and fun to do, look silly with the main subject sitting still and figures frozen in place...
I did build a UH-1D with a powered main rotor and plastic disc TR, and it was a royal PITA... I never did get the rotor centered EXACTLY (the blades weren''t balanced, nor was the rotor shaft centered, and there was a subsequent vibration that made it bob & weave all over on its stand, like the pilot was having a seizure or something... Even after I removed it from its stand ( I had it posed about two inches off the deck with the troops un-azzing it) and stapled the skids down to the ground material, the thing still bounced around & up & down (albeit not as bad, but still too much to run the motor for longer than it took to convince a viewer that I'd put a motor in it.)...
I never could run it fast enough to get it to look like it was moving in a photo either... Woulda shook it apart, lol...
In this month's FSM, there's an article in which the modeler "blurred" a kit prop for a Mustang, and it looked pretty convincing in the photo, but I think a helicopter main rotor would be just too big for that technique to look "right"...