http://www.paperspecs.com/why-print-colors-hard-to-get-right/
The link above is the long explanation on why I double check by the numbers, not just by sight, any colors or corrections made to photos for print designs. We try and come as close as possible to the color you are working for when it comes to full color printing.
No matter how much you calibrate your monitor to match, ink is still different from the colors produced on a screen. The numbers don’t lie AND your monitor also can’t take into consideration the paper you are printing on.
If you are using a pantone spot color, of course your color will be exact as spot colors are matched exactly to pantone specifications.
A Pantone swatchbook is also a useful tool for reference even on full color projects. So are color match proofs. If you are picky about your colors, a color proof should be done prior to printing for a full color project. Often created by a sublimation printer or inkjet printer, these color proofs are used to make any final adjustments or calibration to the press before doing the full rung.
Color proofs are sometimes not an option with cheaper print runs. Some bulk printers won’t do them because they create a project delay while being approved and create an extra step in the printing process itself.
It doesn’t hurt to work with someone who is familiar with the process and the various factors that affect the colors of a printed piece to help you achieve the look you are going for either. 😉
More info on color proofs: http://en.wikipedia.org/wiki/Prepress_proofing