Your going to get a lot of responses on this thread.
If you casings are dirty, you can scratch your dies or embed carbon/grit into the steel, even the carbide ones. Then you will end up with all cases with the same marks or scratches.
So we most all clean to some degree. Washing can do a lot. Tumbling in a 'rock tumblers' with various media works. Shaking in one of the vibrating tubs with corn cob or walnut hulls does a good job.
The simple process of wiping down the case with a rag does well.
I have found that for straight walled cases, little cleaning is needed more than removing grit from the outer surface.
However, I have also found that bottle necked cases will build up a crust of carbon on the inner surface. I know that I don't need to but I always size, punch the old primer and clean. The sizing operation breaks up some of the carbon coating and cleaning after sizing cleans it out along with cleaning the primer pocket. OK, first cleaning with corn cob in a shaker, size/deprime, tumble with 'Simply Green' and steel pins in water to remove lub from the outside and carbon from the inside and then trim/chamfer. Now I can feed my brass to my progressive loader. I always trim to length before every loading but that is another subject and I know is not always necessary.
So, is it necessary to tumble? No. Do what works best for you.