GIMP Plugin Registry

Hello all. So i just downloaded the plug in to allow me to convert RGB images to CMYK in GIMP. But i have many questions about this. WHich choices are best to choose when doing this? What file type should i save it as to get the best quality? Some of the images i'll change will be used on our website, and some will be used to make 4 color card flers (postcards). Please help.