Don't use IAS, as it is subject to system indication errors on the aeroplane but those don't exist on the blades.
You can use any of CAS, EAS or TAS - so long as you use the same for both forward velocity of the aircraft and rotational speed of the blade, you'll get the same answer.
I think that if I was working it out as an aerodynamicist - which I have had reason to do in the past, I'd use TAS. The reason for that is that TAS is easy to work out from geometry and engine speed of the blade, and similarly the path to reduce IAS to TAS for the aircraft is straightforward and well understood and allows you to remove system indication errors and compressibility in one place.