Your printer probably want cmyk stuff. Otherwise he has to do the conversion, without you controlling it (or being happy with it). Photoshop does a rather good job at it. I usually do the conversion as the last thing. When colors are really messed up, you will know that those particular colors can not be printed, you can not counter that by not converting. The printing will be done in cyan, magenta, yellow and black, so that is how it has to be done. Good luck!