It is a steady myth that you need to convert every work for print design from RGB to CMYK color mode. Once I forgot to convert my work to CMYK for printing in Konica Minolta C6500 print machine and by chance I got better results. Image->Mode->CMYK is possibly most perplexing command in Photoshop. Because each print device has its own color profile and moreover even own adjusting algorithm for color conversion.

By converting RGB image to default CMYK profile you decrease quality of print work because in print device it will be converted second time anyway adjusting image to concrete color profile and by any conversion you lose valuable image data. Sometimes print machines have better technology/know-how in conversion images from RGB to CMYK and preparing them for print.

You have to know that ideally the new color profile for print machine should be created quite often.  Each print job needs a different CMYK profile depending on rubbers, inks and paper type used, pressure, density, print speed, temperature and relative humidity of the press room. Al these parameters have an effect on dot gain and so they can‘t be calculated by Photoshop or other software. They have to be adjusted, ideally, just before press run.

Tip: Preferably color conversion should be made by prepress department of printer company. Contact the prepress department at the printer you are going to use and ask them what they exactly want. Avoid printing in companies where staff did not hear about color profiles at all (I met such people). And don’t forget to use at work good monitor and calibrate your screen, better by hardware not software approach.