Well you want to see what you are doing. RGB shows more colors than you can print. Theoreticaly when if you have cmyk profile from your printer and calibrated monitor you should see somehow accurate representation of how will things get printed.

Not a color expert, but every color encoding scheme is going to have a slightly different gamut. Meaning, CMYK can encode colors that RGB can't, and vice-versa.

Negative color values can be used to cleanly represent colors outside the standard gamut. But I'm not sure that CMYK even has anything like a standard gamut associated with it - so the details of that "conversion" might depend on what the CMYK data is for.