I don't know what program your using but if you want a close to accurate reproduction of colors always work in cmyk. By the way color various per pc, mac , etc especially monitor colors.
That would be true for a regular 4-color printer, but sometimes printing houses use 6 or 7 color large format printers. In which case you should always give them RGB and let their own machine convert it to CMYK, thereby giving a much higher range of colors than if you just gave them a CMYK file.
Ok, how can there be a greater range of colors with RGB files instead of CMYK considering there is no true black with RGB.
And far as letting there machines convert your files to their format be it cmyk or rgb, then saturation and correct color is kind of hit or miss. Also 6-7 color printers use special inks like spot colors or chroma color inks. Being a graphic designer I have to deal with color correction and print issues all the time. Not to dog anyone here just giving my two cents.
You're not the only designer on these boards

- maybe this will clarify. I found this little snippet off the web, some printing company who wrote up a really basic FAQ for customers:
[begin]
The real difference between RGB and CMYK.
ADDITIVE PRIMARY COLORS The red, green, and blue (RGB) components of light are called additive primaries. The display of all monitors is RGB. The additive colors are transmitted from light sources through the monitor.
SUBTRACTIVE PRIMARY COLOR A color system in which pigments, cyan, magenta and yellow of printing inks, are mixed to form other colors. Subtractive colors are created by light being reflected rather than emitted. Because of the difference between the way additive and subtractive colors are seen, color variations will be experienced when viewing colors from different light sources.
RBG COLORS vs CMYK COLORS All computer monitors create color via RGB components.
The RGB mode has many millions MORE colors than the CMYK spectrum. Too bad we can't print RGB!!"
[end]
Obviously you have more colors available when you use RGB. Some printers can reproduce a larger number of colors than others. A crappy little inkjet will have a limited gamut and our 500,000 xerox 4-color printer has a much larger range (redder reds, nice deep blues, bright greens, etcs). It's higher because we use better inks, as do some professional shops. If we were to send stuff as CMYK (from photoshop lets say), we would not be reaching the potential of the machine.
Maybe we are on 2 different pages here - and maybe this discussion could benefit some non-designers.