For thread cutting, you will end up using several different spindle speeds. It can also be nice for different tooling types like drill bits and cut-off tools.
Are you worried about setting up the digispeed chart? You can get away with only specifying 3 values and speeds. I believe TCNC will figure out the values in between, but it will be a linear calculation. I also believe that a more detailed map will give you better connection between the speed you choose, and the actual speed. I have not tested this enough to say exactly how it works.
My advice is to just take some "air chips" and test it. Did you order the Digispeed XL?