It's debatable what the stock spring cups were intended to do...some say they act as a spring damper, some say they are an oil shield, some say they are a spring locator device, etc. I don't know what their design intent was, but I know for sure they are about .030" thick and if you reuse them they will act as a shim and increase the spring force.
If you use CC980 springs, for example, which are advertised at 307 lb/in then using the .030" thick cup will cause an extra 9.21 pounds of spring load (307 times .030). Which is fine if that is what you want...but if you are running around 11.5 or slower then it is my opinion that CC980 springs without the stock cup would be preferred...that's what I run too.
12 springs cost about $40, a spring compressor from Jack Cotten costs another $40. Did mine with his tool and it took me about 2 hours and I was very slow and meticulous...check out gnttype.org for the detailed process.
Regards, Bob