I'd like to hear one of you colour "wizards" explain why anyone would design in RGB when the application is a CMYK printer output. If your system is fully colour-managed to deliver the maximum CMYK gamut your printer can output, that means you have custom ICC profiles that are in CMYK space. Shouldn't you be sending those ICC profiles to your clients so they can design in your printer's gamut?
Common RGB color spaces (sRGB, Adobe 1998, ProPhoto, and a few others) generally provide more universal use applications than a device specific CMYK color space. Nowadays, a vast amount of graphic production is targeted to both screen display of some type AND print of some type. A smart design shop can create a single file in RGB and have both targeted end-use result with very similar colors.
However, it is misconception that RGB is a larger color gamut than CMYK. It depends upon what RGB space one is considering against what CMYK space one is considering. For example, both Adobe 1998 and sRGB can be deficient in cyan colors against certain Epson eco-sol printers.
Designers need to have any and all working space profiles and output space profiles at their disposal so they can create colors which match among their intended targets. Unfortunately that's usually a lowest common denominator such as sRGB for websites on older monitors and CMYK only printers. It's then often required to sacrifice cyan gamut that a good CMYK printer may be capable of but sRGB (even Adobe 1998) can't reach, and also sacrifice warmer colors that CMYK might lack but the two common RGB spaces can reach.
Another very important factor of the common RGB color spaces is the fact that they are neutral as far as black through the gray scale to white. Knowing that 3 equal numbers is gospel and that carries a lot of weight wherever the file is to be reproduced. If a color is specified as 128, 128, 128 the color should result to neutral gray on any device. What are definite neutral values for CMYK on any device? Or black values for any CMYK device?
Knowing common, ubiquitous values for neutrals can easily reveal if a workflow and print process is controlled or not. The G7 method (and others) can certainly help with the task as well as far as values for CMYK work.
Shouldn't you be sending those ICC profiles to your clients so they can design in your printer's gamut?
Some print machine companies provide their CMYK ICC profile. For the space to be valid a print shop would need to be calibrated to the same spec as when the space was created and published. The first test would be to learn how neutral gray prints from the given print shop and this is but one way how some print vendors are tested and qualified.