Yesterday I heard a "graphic designer" say, "Really there's no reason why you should ever design in CMYK." I almost burst out laughing.
For those of you who may not know, or haven't figured it out yet, I have a heavy printing background. I studied printing prior to studying visual communication. And all of the knowledge that I have gained about printing has helped me immensely in becoming a better designer... that's another post though.
These differ substantially from the primary colors of light, otherwise known as RGB (red, blue, green). CMY are considered subtractive color and RGB are additive color. If you add R+G+B, you get white. If you subtract 100% C&Y&K, you get white. If you add C+M+Y, you get a muddy brown. Black is the key (K) to achieving a better spectrum.
If you are looking to design something for the web, use RGB. Anything for print should be CMYK. I realize that pre-flight software will adjust colors and even your printer will print and RGB image, but shouldn't you try from the very beginning to get the best color in the end?
My guess is that at some point in time the logo you designed will in fact, be on press utilizing the CMYK color space. You should plan accordingly.