This is not the be-all end-all guide for color modes, just best practices from my experience designing for the printing industry.
The only thing that matters is the output.
The simple rule is: web=RGB, print=CMYK.
If you are designing for print, what will the final output be? What kind of press will this be printed on?
Offset presses print from plates. Process printing uses cyan, magenta, yellow and black, or CMYK. It is an offset press that will print spot colors or varnish. Offset presses are for large print runs of single sheet printing like letterhead, cards or fliers.
Web press: CMYK
Newspapers and magazines are printed on a web press.
Ink jet: RGB
Desktop ink jets utilize the RGB data for printing. If you send a CMYK document, it will actually convert it to RGB so it can process it. Converting the color twice in this case will likely print unexpected color.
Digital: CMYK or RGB, not both.
Digital presses have a RIP that can process color data and make it printable. It can process RGB data and make it printable and it can process CMYK. If you have RGB and CMYK data on the same document, the RIP gets confused as to how it should handle the data and something will not look right. Pick a color mode and stick to it. Digital presses are similar to a color copier in function. They have a set number of colors, no spot inks and they print on very limited paper stocks. Formats larger than 12×18 inches are rare and costly. Digital presses are cost effective on runs up to about 500 before you are better off printing offset. If you are printing variable data, like addresses or names, this is what a digital press was made for.
Large format: RGB
Large format printers are ink jets. They usually have seven or more inks which allow a very wide color gamut. If you narrow your document down to only CMYK, you will not be using those seven inks to your best advantage.
Why use CMYK?
You know what your color breakdowns are and you know what the end result will be. You are able to monitor your ink mixes exactly throughout the design process and there are no surprises.
If you design in RBG and convert to CMYK, no matter what conversion settings you use, your colors will shift. There is no black in RGB, so anything that is meant to be black will be washed out and probably print on all four plates. If you have small type, this will be a huge problem. Press registration will have to be dead on to match up all four plates of tiny type to make it look black and that never happens. With CMYK, you can use pure 100% black for the type and avoid any problems.
RGB has a much wider color gamut than CMYK. There are millions more colors available on screen than can be mixed using four inks. Any out of gamut colors will be estimated to something that is within the printable range. If you are using bright colors on screen, specifically greens, blues and purples, you are probably going to be disappointed. This applies whether you are converting final images in Photoshop or exporting a PDF with print settings.
The only drawback I can think of to designing in CMYK is the limited filters in Photoshop.
My monitor only shows RGB.
RGB is colored light, CMYK is colored ink. Any screen shows RGB because it has to. There is no ink in your monitor. However, a good calibrated monitor will show simulated CMYK. Working in CMYK mode will not display electric blues or radioactive greens. The final printed result will be much closer to what you see on screen.